I know it's not what you want to hear, but I have TWO of these and they both work great. The only time mine stops is when the pump jams occassionally. Then it's just "tap,tap' on it and away we go.
It sounds like your air tube might not be totally clear. That's what provides the bubbles, so it needs to be clear and, of course, above water level. If you blow on it, like a straw, do bubble then enter the collumn? It should not be hard to blow bubbles. What are you using to provide water to the skimmer (through the 1/2 inch rigid tube that stands up toward the right of the pump. On one of mine I just use a gravity feed from the tank. On the other I have a small powerhead connected to it, feeding water from the sump.
Once it's being fed water and air, and the pump is running, the only adjustments are with the flow of the water leaving the two output tubes. There are baffles on the top of each that allows you to restrict the output (bubbles rise higher in the chamber) or open the output if the bubbles rise so much that water is collected in the cup instead of nice dry foam.
If not write again,