Remove the collection cup, and then play with the air valve until you get a good solid set of bubbles coming up and just ever so slightly over the black cap, and collar that the collection cup sits on.
If you run the air too much heavier you'll end up with a cup full of water within a couple hours.
If its set right they seem to work really well. Ive seen them set up in some LFS's and producing some really nasty crud... and although mine is still new, its already starting to produce some good results.