After experiencing button failure on my 18650 noEgo (Same as the E-Power), I decided to take a pipe cutter to it in the interest of science. Here is what I found out:

The center plastic ring is glued to the housing with what appears to be super glue. This glue is very brittle and the plastic ring is smaller than the inner diameter of the housing. The plastic ring can become loose and rotate inside of the housing. If this happens, while even the "hard plastic" button is being pressed down, it can cause the button to stick or not fire at all.
The plastic ring in the 18650 model is pretty thick, so after my replacement noEgo arrived I went ahead and drilled a small hole in the outside of the housing on the opposite site of the switch. I put a screw in the hole and tightened it down. Now the plastic ring can't rotate and the switch will always stay lined up properly.
If the manufacturer would use a larger diameter inner ring and better glue, this modification wouldn't be necessary.
So far, my new noeGo is working fine and I'm pretty sure this would work on any E-power with a switch problem. Just be sure to drill a very small hole that a very small screw can fit. Directly behind the button is the PCB. You really don't want to screw through it.
This is a fix that can be applied before the problem happens. You could probably rotate the plastic ring to re-align the button after you drill the hole, if yours is already malfunctioning.
Good luck and I hope I posted this in the right place. I have been lurking a while and am just now figuring out the posting etiquette.
-BlutO

The center plastic ring is glued to the housing with what appears to be super glue. This glue is very brittle and the plastic ring is smaller than the inner diameter of the housing. The plastic ring can become loose and rotate inside of the housing. If this happens, while even the "hard plastic" button is being pressed down, it can cause the button to stick or not fire at all.
The plastic ring in the 18650 model is pretty thick, so after my replacement noEgo arrived I went ahead and drilled a small hole in the outside of the housing on the opposite site of the switch. I put a screw in the hole and tightened it down. Now the plastic ring can't rotate and the switch will always stay lined up properly.
If the manufacturer would use a larger diameter inner ring and better glue, this modification wouldn't be necessary.
So far, my new noeGo is working fine and I'm pretty sure this would work on any E-power with a switch problem. Just be sure to drill a very small hole that a very small screw can fit. Directly behind the button is the PCB. You really don't want to screw through it.
This is a fix that can be applied before the problem happens. You could probably rotate the plastic ring to re-align the button after you drill the hole, if yours is already malfunctioning.
Good luck and I hope I posted this in the right place. I have been lurking a while and am just now figuring out the posting etiquette.
-BlutO