Hi Paul,
A couple of thoughts from the sublime to the ridiculous… take your pick 
You mentioned having this problem back in Nov '24 in a reply to someone else’s problem:
Are you pressing the string at the right place- right up as close to the fret as you can get? Even the lightest of pressure on the string should move it downwards. It only has to touch the fret (metal), and the string should never be pushed down to the fretboard (wood). If it does, you will be out of tune (sharp). Most beginners press far too hard and often away from the fret.
Slide guitar!
bottlenecks don’t groove
If you do pursue the rubber caps route, go for black caps, and let me know how you get on. I could do with another verse 