Is your POW in a bottle or a pot?
You have to ensure that the threads on the bottle/pot and lid are clean of product so that it's sealed when closed.
It's not something I've come across yet though.
are you sure it hadn't started to cure?
when you apply it do you keep the pot out of direct light?
Yes it can start to cure like that... any light can start to cure it... if you're taking a while to apply the gel/pow then loosely pop the lid back on the pot to protect it from the light, either that or place the pot behind the nail lamp so it isn't in direct light.