Tell us a little bit about your setup. What size tank, what kind/size of skimmer and is it external or in-sump.
Often times there is a break in period with a skimmer, it can take a day or two or even a couple of weeks before it starts pulling crud out of the tank the way it's supposed to. Usually adjustments will have to be made by raising or lowering the water level in the skimmer. Certain skimmers run best with the water level set at a certain height...etc.