By foam I hope you mean just putting out micro bubbles, if so, how long has it been on the tank. Also, when I first set mine up I was missing the o-ring that fits around the cup so you can adjust it up and down. So I had the cup sitting all the way down. The skimmate was really wet with it all the way down but it would fill up about 1/4 inch a day (I have only two small gobies, so bioload is pretty small), I slowly bumped up the cup until it started pulling nice skimmate. What pump do you have on it? Also, have you tried to clean out the injector nozzle, I would imagine if you had a blockage here that it would cause it to not skim properly. Hope this helps.