Is it an in sump, hang on back or external? Skimmers have a break in period usually takes a few days. Once working it should be making bubbles and creating skim mate that fills the collection cup. You can change the air input to skim your tank to produce wet skim mate or dry bubbly skim mate.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"