Problem with Euro-Reef CS-6-2+ not sucking air.

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I've had a Euro-Reef CS-6-2+ for years and if has always been reliable if not my ideal skimmer.

However for the past month or two the skimmer will not suck air when the pump is started up. The pump will turn and it will fill the skimmer chamber with water but no air will be drawn in. If I blow on the airline tube then the air will start to be drawn in and the skimmer will skim normally.

I have taken the skimmer and pump apart and cleaned everything twice. The needlewheel and impeller look OK. The volute and air intake nipple look OK. Of course I clean out the silicone air tube regularly.

The only thing I have noticed is that when I go to clean the impeller the impeller will not come off the ceramic shaft. It spins freely on the shaft but can't be removed from the shaft. I am not sure if it is supposed to come off the shaft or not.

Any ideas on this?