Make sure the air intake is open. Also make sure the discharge pipe isn't under water. If it's installed in a sump the water level should be even with the bottom of the discharge port.
Start with outlet valve open all the way. You should see bubbles inside the tower. Slowly close the outlet valve until the water level inside the skimmer is at the top of the gray box.
Make sure your pump is powerful enough for the skimmer.
First and foremost, make sure the venturi isn't clogged with salt, buildup or water. Make sure no water is getting into the airline. That will pretty much defeat a venturi. Take the valve off the end and blow air into it. You should be able to see a lot of bubbles pour into the column if it's not clogged. If it's stopped up you can unhook it from the tank and run some warm water through or run some vinegar through. The acetic acid in the vinegar will clean it up w/o messing it up. Be sure and rinse it thoroughly, though.