If you play either Melee or PM more than the other, the one you play less feeling less responsive/awkward is a pretty common thing. idk about differences in button sensitivity/responsiveness, but ik one factor that plays into this is the difference in character models and the cameras. Melee has smaller models (visually; the hurtboxes are the same), but has a more zoomed-in and faster moving camera. PM's character models from Brawl are made a bit larger to match the hurtbox more closely, and the camera is a bit zoomed out and calmer when it pans around to follow characters. Because of this, while characters may be about the same size on screen visually, they may technically be a bit bigger/smaller than one would assume, and the slight difference can end up throwing players off more than one would assume. This would play a part in things like sweetspotting ledge and stuff, since the character may go off screen at slightly different timings/distances between the games (just an assumption, but I think I've read this elsewhere).
On another note, that shouldn't affect certain things like inputs for basic tech that should just be relying on muscle memory, but I know tons of people that find themselves flubbing the silliest things, like full-hopping 1 in every 10 or so of their wavedashes (from non-scrubs). That'd mean that they're probably pressing L/R too early (before the jump crouch animation ends), but...iirc the timing between Melee/PM for basic tech like that is the same. Weird pseudo stuff probably playing a part somewhere in this >>
I'd just keep playing both consistently until PM gradually starts to feel less awkward. That, or consider finding a different main between Melee/PM, so the slight differences don't throw you off if you leave one or the other for a while.
