Make sure after you do any adjusting you wait at least a half hour. I didn't think I had mine set right, so I turned the air intake up a little bit, and walked away cuz I didn't have any bubbles comeing over the top...
Well after about half hour, my cup was full...just remember there is a "break-in" period for skimmers, so what works today will be different tomorrow, until you get it adjusted properly....
I try to set mine so that the bubbles are about 1/2-3/4 way up the tube, and then leave it alone.
Also, when you add water, do a water change or add supplements to the tank, it will also affect how you skimmer works...
Hope this helps!!