The first thing to do is to double check the manual for the depth it should be in the water. Then (of course) make sure it is at said depth.
Next, with it running, close the adjustment valve. Now, before doing this, keep your hand near the power so you can pull it if things go crazy. After closing the valve, the water has no choice but to come out the top, so you should see the water start to rise and flow into the cup. At that point, open the valve back up all the way. If this test succeeds, then your skimmer pump is working properly and there are no water leaks.
Next, check for bubbles going into the main chamber. The air hose going into the pump should be above the water line in order for the pump to pull in air. If you plug said hose with your finger, you should notice the change in the water in the chamber.
If those two tests have the expected outcome, then your skimmer is technically functioning.
So now, close the adjustment valve to the point where you see the bubbles rise in the main chamber. Dial it in to the point where the bubbles are half way up the chamber, then every day, dial it in tighter and tighter. Eventually you have have a small amount of bubbles going into the collection cup.
NOTE: Looking at an image of this skimmer, it may not have a valve. So the first test would be to simply cover the vertical tube with your hand instead of closing the valve. If this is the case, then the problem is likely the depth of the skimmer or get a valve for that tube.
The main principal to get the bubbles into the cup is to increase or decrease the back pressure inside the chamber, allowing most of the water out of the outtake hole, but with just enough back pressure to cause the bubbles to climb.
All that said, what do you have for fish in the tank?
