Is the skimmers reaction chamber(cylinder)filled with bubbles???If not you have an air induction problem.This could be as easy as just readjusting the air valve.Or maybe a good cleanout of the pump.If the chamber is filled with bubbles,but no foam is rising to the cup,it could be that the water in your tank does not contain enough dissolved protiens and waste to be skimmed out.In other words,Your skimmer has done it's job,and your water is in good shape.The size of your tank,and ammount of fish in there also has alot to do with this.A very heavily stocked tank will need MORE skimming than a lightly stocked tank...Try turning it off for a few days and restart it.Then see what it does...I only need to run my skimmer a few days a month.