Do you guys ever have these moments where a particular piece that you just learnt and have been playing well for a few days suddenly becomes blank? I recently experienced this practising some licks where I suddenly couldnt play something mid-practice. I’ve not been able to play it as well ever since.
I’m wondering why it happened and how I can identify my mistake. If you guys have some thoughts please share 
Yes and it’s not unusual. To really learn something it need to be ingrained in you memory and
a few days isn’t long enough to do that. The solution is to go back and learn it slowly again.
Think of all the things your brain encounters in a day and most of it is not important. So you brain
takes the garbage out and sometime things go out with the trash.
Sure, happens to me quite often, especially when its freshly learned.
I just have to keep going over it and over it until its ingrained in my brain and
finger muscle memory.
Don’t despair, just keep rehearsing it, over and over and over and…you get it.
It always seems to take me a few weeks of practice to play a piece of music all the way through without losing my place in a chord progression. I have to review lessons several times to make sure I haven’t forgotten anything.
Sometimes that happens to me, and not just with guitar but with anything I am learning. You say that even after you got past it a little bit you feel like you have regressed. I would actually set it aside for a day or three and come back to it refreshed. I think sometimes my brain is sending me a message — “hey, back off! You’re force-feeding me too much! Let me digest a little bit!”
@Dave911 I was thinking about doing this too! Maybe I’ll take a break and practice something else for a few days.
I have this happen from time to time even on songs I’ve been playing for some time. Funny thing is if I go back straight away and try to play it, the blank stays. But if I wait a little while, maybe 10 minutes or so, then it’s all good again.
+1 on what @tony said. I was attributing it to me being one of the “old dogs”, but maybe it’s just “new lesson overload”…
I’ll back Tony up as well on that. In fact it happened only yesterday. A song I’ve been playing for a couple of months, I got the a certain section and “blank”. Tried again and it was a no go. I ended up looking in the book to see what it was.
The biggest challenge I’m afraid you’ll find as you progress is actually retaining what you have learned. You’ll find it difficult to remember licks, songs etc. unless you are practicing them regularly. I’ve probably forgotten more than I now know.
Welcome to my world 
My post-covid brain is having an even harder time now too. It’s an odd and unsettling feeling when something you felt you were good at and knew pretty well is a bit confusing again.
Yeah, I’ve tried to claim “long-Covid” for my brain-fog a few times. My wife is having none of it! She insists that I wasn’t especially clever before Covid and I’m still not!
(Not trying to say Covid brain-fog isn’t a thing though.)
An update: I took 2 days off from the song and been practicing it past hour. Somehow I am playing it even worse. Been trying to observe what I’m doing wrong, see if I can improve.
What song is it your having trouble with? Is it the timing or
chord changes? Give us some hints we may be able to help you
figure out what has changed.
Crossroads by cream. Trouble picking the beginning of the turnaround.
Slow yourself down and play it correctly. If you keep trying to
play at that speed you’re re-enforcing the mistake so you’re
in graining the mistake into memory.
Go back to the lesson and start over and learn it. Every thing
you repeat a mistake without correcting it, it gets harder to fix.
Hope this helps
Exactly THAT section in crossroads had me cursing and getting frustrated for a long while.
Then I’ve gotten into a zone where I did an “acceptable variation” and after not playing it for a longtime, I forgot how to play it 
on the learning, expect to need time
practise-sleep-practice-sleep… too much without your brain rehashing all that won’t teach you much muscle memory 