Do you have another battery to compare the button action to?
Is it mechanical?: Are you getting that soft "click" and slight tactile feel when you push it in, and more importantly now, again when you release it? My buttons have a little bit of play between the button and the battery case. Not much, but if you just move your thumb back and forth across the button - without trying to press in on - you should feel the button move around ever so slightly.
If not and it sorta feels like the button is sticking "on" and not releasing it makes me wonder if something (cough, cough e-
juice) got in/around the button that's causing the problem.