The biggest one for me is probably heelflips, as a teenager they were an absolute stock trick for me and I had nice popped ones, could also do doubles without too much effort.
Now in my mid 30s, since I came back to skating I just can't get heelflips consistent apart from rolling fakie (even then I have some pretty off days on those), my regular ones are untidy and very inconsistent and despite many attempts I just can't get them back to where they were.
I attribute this to a couple of things - learning to kickflip last year has changed my body and weight distribution over the board a bit when i set up for flip tricks, also to moving up to skating 8.25s... I used to do heels on 7.5 setups and i think i struggle to get the same results on a more mellow wider modern setup.
Most of it is mental though, I know I am capable of doing them well, it's just having that self belief and having the discipline to put consistent time in every session to relearn the trick.
Since I learnt a bunch of kickflip variations I just don't focus on heels as much as when those were the only flip tricks I had, and I'm sure that's a big part of why I haven't got them back.
the "flip" side of all this is that for the one or two tricks i've lost, I've learnt 25-odd new ones in the last couple years... try not to get hung up on individual tricks when there's so many out there.