I learned a clever trick from the folks at MAS epoxy. When the pump stops working, dunk (not submerge) the container in hot water. It may take a couple of water changes for the heat to permeate the container, but once warmed up, it softens the whole works and voila, the pump works like a champ again.
PS, I'm using MAS F.L.A.G. epoxy because it doesn't need to be thickened for use as glue. I've only had to use the hot water trick once. I've never had it happen with WEST epoxy which is much thinner.