How long have you had the skimmer? Did it ever used to work and just stop? Do you see any micro bubbles? Do you have your hands in the tank a lot (re-arranging the rock and such)?
I had a SeaClone 100 that only worked for the few couple of months. Now I have an AquaC Remora and it works great. HOWEVER, if I'm re-arranging the rock, feeding, or anything to disturb the water then I don't get much in my collection cup.