Happens with mine as well; also hayward. I found that if I "break" the seal while the pump is running, then turn off the pump it comes off much more easily. That's probably risky business, though. The printing on the cover suggests using a piece of wood (2x4 I guess) in between the two ridges to break it loose. It also warns against over tightening. I've found it leaks air in if I tighten it less, though.