I use one and have for a while if your bio load is low you will have a hard time getting skim out of it just like any good skimmer it should produce more bubbles after feeding and also at night as the temp drops a bit the water molecule move to wards each other. Unlike anything else water is the only molecule that will condense and then expand when cooled. so I would say your skimmer is working properly you do know it is rated for a 250g tank.
What size of tank are your using it on? If it is a small tank I would recommend that you get some more stock before you condemn the skimmer, by the way it does take time for these skimmers to fully break-in mine worked very well after the 3rd month I had it and after my first maintenance cleaning. ever since then it has pulled realy nice black skim out of the tanks it has been in.

Bill