Which app are you using?
Discussion
Right now I switch between Damus and Primal because I can’t decide which one to use.
I don't use iOS so I haven't tried Damus and I rarely use Primal. On Amethyst, which is Android, sometimes I've had to press Follow a couple of times to get it to stick.
If that doesn't work, maybe you could copy their npub, close the app, reopen it, paste in the npub and try to follow them again.