You have to keep it cleaned regularly or else this will start to happen with the 18650 and the 14650 version.
What i find works good is 91% isopropyl alcohol and a q-tip... get the q-tip nice and soaked in 91% iso, take the q-tip and rub the iso all over the sides and top of the button, now while the button is still 'wet' from the iso give it a few good firm pushes, them use the dry end of the q-tip to wipe away any excess iso from the sides of the button, repeat as much is needed until the button stops sticking/getting lower.
I've had my e-power about a month and I do this to the button every couple of days, haven't had an issue... maybe it's more of a preventative measure but I started to do it because the button was showing signs of sticking on day 3 after owning it.