Maybe the B string is falling under one of your knuckles. Things to try:
re-position your index finger slightly so the string is under a fleshier part of the finger
roll the finger more on it’s side, and curve it
(last resort) focus on pressing index finger down harder over B string - without increasing pressure and tension in rest of the hand!
Thank you so much for the tip. It seems to be the right one for me. I’m now getting the B to ring out with the other strings quite nicely! Well most of the time now anyway.
Cheers 
In the lesson Justin says that those with an electric as well as an acoustic guitar should practice the F chord on both, to work on strength as well as technique. I don’t have an electric guitar, but I’ve found that tuning all the strings down half a step on my acoustic achieves a similar effect, as it lowers string tension. I just thought I’d share.
Welcome to the community Pi. (Domen)
The Barre F chord is a movable chord so instead of tuning down you can move it up to the 5th fret. This would be an A barre chord. When that become easy move to the 3rd fret, that’s a G barre chord then down to the first fret, F chord.
Save re-tuning every time you want to play in standard tuning and you learn 2 other chords at the same time
This video got me to focus on my wrist. Justin mentioned something in the video about not having a bent wrist. So how much is to bent and what injuries can you get from it as I noticed I sometimes bend very much.
If your wrist angle is closer to a 90 degree bend than being completely straight then it’s probably too bent. The injuries you can get (such as carpal tunnel syndrome) can affect your whole life, not just your guitar playing, so it’s well-worth spending the time to get a good, comfortable wrist position.
It may be worth taking a look through this thread: Wrist & forearm pain
Thanks for the answer. Seems like some famous players plays with their fretting hand pretty bent like Hulkoff and his band members. But will now I will check out the thread you sent.
You can also put a capo on the first fret after detuning to bring the guitar back to normal tuning.
Additional advantage is that the frets are a little closer, which makes things easier.
Loving the course so far but really struggling with F Chord. Specifically, the B string for me is always muted. Looking closely at my index finger position, it seems that I am barley pushing it down compared to the other strings as it seems to be underneath the joint crease in my finger. Tried moving it up but I feel like my index is then way to high above the fretboard, moving down then leaves it difficult to fret the 6th string.
Curious whether anyone else has ran into similar trouble and how they fixed it
Lot’s of people have trouble with the B string.
I just did a search in this thread for “B string”, and got more than 10 hits.
There are multiple suggested remedies as well.
The consistent solution to the B string is to gently pull towards you with the fretting arm, stabilizing with the strumming arm. This leverages the finger down onto the thinner strings.
Justin goes through this in his lesson. I found it hard to do, not physically, but to add that little bit of consciousness to include the slight pull. It is slowly starting to happen naturally.
I’m still working on consistency myself, but the tip that worked most effectively for me was the slight roll to the side of the index finger. If my finger is flat on the neck I have the same issue you describe, when I roll it slightly is the bony bit of my knuckle on the b-string which means a better connection and more pressure down.
I was not new to guitar when I first started with Justin’s lessons. I really thought that I knew how to play a barre chord. So when this portion came around, I was kinda cocky that I will breeze through this portion easily. However, when I started to play the barre note for note. To my utter horror, all this while my thinnest two strings were being muted. This was such a horrible surprise to me. Now, I have to relearn this beast. Agh!
Nonetheless, I’m glad to learn from the master incarnate himself. Let’s go!
When I was just starting with the F barre learning, My wife and I visited a friend of hers, whose husband played guitar well enough to have been in a few casual bands.
We were playing a little (because that is all I could do) and talking and I was discussing how much trouble the F barre was to learn.
He says “your teacher really wants you to get all the strings to ring out on a barre cord?” 
He couldn’t and realized just then he probably should! Glad I could teach him something!
I think subpar barre cords are common. But as Justin says “practice makes permanent, so practice perfectly “ and we should not learn sub-par skills, to the best of our ability…
I can play it 1st try except from the 2nd thinnest string . The B string .
Any advice ?
Try the rolling of the index finger over a bit technique. Everyone’s fingers are different so just have to adjust until the strings all line up with a ‘boney’ bit, best you can.
It can take a while so don’t beat yourself up. It will come.
There is actually another (and easier) way of doing the F chord. You put your 1st finger on the first fret of strings 1 and 2. You also put your 2nd finger on the 2nd fret of the 3rd string. You put your 3rd finger on the 3rd fret of the 4th string and don’t strum strings 5 and 6. This type is easier and makes the exact same sound as your version. Try it!
Hi all, thanks for the replies and tips! Always nice to know I’m not the only one struggling
I had taken a look through the forum here and others online and couldn’t really find anything that resonated in a way that helped. However, through a little trail and error I have managed to find something which has helped me play the chord much more consistently and I wanted to share it in case it helps someone else.
So, in the video, Justin talks about ‘rolling’ your index finger; not totally to the side but so that it is not flat. I felt as though I was achieving this at first as my finger wasn’t flat but also wasn’t completely on its side. However, regardless of how hard I pressed, I just couldn’t get the B string to budge.
After pulling my hair out for the last few weeks trying to crack this, I recently started to try placing the ‘E Shape’ down with fingers 2, 3 & 4 and then laying my index down flat first of all. When I was fretting the chord, I would press my index finger down but also slightly up as well, not so that it moves from the fret but I can feel an upward pressure - and it works!
To me, it looks almost the same as what had been doing previously but I imagine it must have something to do with how the strings grip my index finger that is allowing me to hold them all down.
Hopefully this might help someone else who is struggling!
When you say “…I would press my index finger down but also slightly up as well…” do you mean that you’re pressing it slightly toward the low E string? I haven’t tried this but it would makes sense that this creates a little more pressure on the B string.
This is the most frustrating chord ever. OK, I don’t have long fingers, and I’m struggling so much with F on acoustic. First issue: my pinkie keeps getting flat and muting G string. So I had to do some practice (without index finger on 1st fret) just to get all strings ringing out. Then when I get that going, I put my bar… And no matter how hard I press or rotate index finger as Justin showed in the video, I cannot get E and B strings ringing out. At best E is but B is muted. That’s the best I got….
Banging my head against the monitor did not help either…