I think I know what you are talking about... When I got my V4's I made a post about them, and wrote about how the buttons were a bit different than my other Tama's.
Usually my Tamagotchi's buttons don't go 'soft' or lag or anything after a very very long time. My Keitai's buttons are starting to not work so well, but they are about two years old. The V4's are just... strangely built.