Two common causes of the collection cup filling with water rather than skimmate is 1) needs initial clean, and 2) the air adjustment is set incorrectly...
Ait adjustment is something that needs tweaking untill the correct mixture of air is being introduced into the skimmer..
Cleaning wise, i always recomend before use, that the skimmer be completely stripped down, washed well in warm soapy water, rinsed well in cold running water and re-assembled. Why? Because when plastic componants are made at the manufacturer, the plastic is coated in a protective film. This plays a big part in interfering with how the skimmer reacts to the air/water mixture..
Hope that helps...
Niko