Try using some isopropyl alcohol on a QTip around the button to see if that helps, may just be sticking because ejuice got in there or try electronics cleaner. It may take a few tries to unstick the button. I tried this isoprpyl alcohol method on one of my mods and it worked great- no more sticking!
Hopefully it is just stuck with e juice because I don't know how to take the mod apart to fix the button..
Good Luck