Canreef skimmer woes...

[Son Of Skyline]

This has been driving me crazy for the past 2 days. My relatively new (used) Canreef skimmer suddenly seems to struggle to pump air and the Mag 18 seems to not want to pump any water through it. This seems to have happened after some major work (non-skimmer related) which involved multiple shutdowns of the main return pump (causing high sump levels). The skimmer is in the sump and I never touched it while I was working with the other stuff. It's been running amazingly up until now. I cleaned and inspected the pump and it works fine when tested, and I took apart and cleaned the beckett. Still no good. Any ideas anyone? Thanks in advance..