Nima,
How long has it been setup? Also, take a syringe and sorce water down the venturi tube. This can dislodge and salt buildup inside the tube.
I have used one for years and trouble free until last month. I had to completly disasemble the pump and skimmer to clean it. The pumps output was strong but not strong enough to force enough water through the triple pass tube... There was also a buildup of sludge in the bottom of the skimmer slowing the flow down even more. It looked full of bubbles but produced nothing..
If it has been setup a while, they will slow down after some of the dissolved organics are removed..