OKay,
I have Bubble-magus NAC 9, Its been working fine for a while now. I pulled it out and cleaned it seemed to work okay. Then 1 day I heard rattling of the lid on collection cup, looked in and cup was completely full. So I figured protein skimmer was too deep in sump so I elevated it. To my disappointment it filled back up with mostly water.
skimmer is set on highest skimming level.
also the intake where water comes through is clean.

Does anybody know what might be the problem?