If the bump arm doesn't rise as the tank gets full and has no resistance when moved, it's an "E" clip issue for sure!

Let me tell you what to look for, the shaft in the center of the nest is clipped to the plastic plates (tube sheets) and secured to the lid. If you had the little beastie open earlier this season and the shaft didn't fall out, you probably didn't take the tank lid off the shaft, or the shaft would have dropped down.

It sounds like you've had this apart before, so shouldn't need much guidance - but I'll be here anyway, if you have questions once it's open.