I'd say if the main body chamber is full of white frothy bubbles and some go out the return tube you are getting close. My ER skimmer flows a constant stream of bubbles out of the return side. To prevent bubbles in the tank I used an overflow style foam cylinder over the return tubing. The water flows out and over the foam. The bubbles trap in the foam and dissipitate into the atmosphere. I normally see larger foam bubbles popping about halfway up the neck to the catch cup. When there is sufficient crap in it it will then flow up and over. Never really stays all the way to the top. Maybe your skimmer hasn't been on long enough for it to collect enough gunk to flow over and into the catch cup. If you see the bubbles about halfway up the neck I'd leave it be for about 2 days and see if the cup is filling. If no bubbles at all in the chamber then the airline is definitely clogged, kinked or underwater.