Tunze 9004 not skimming?

[NokesieREEF1]

Hi guys and girls, I've just put my 9004 skimmer back in my tank after running my system without it fir a few months to see what sort of results I would get. However I've now returned the skimmer into my setup and the bubble formation is only half way up the cup. Before I returned the skimmer to the system I totally dismantled the skimmer for cleaning. Any ideas as to why the what was once a brilliant skimmer is now just serving no purpose?

Thanks in advance

Andy