Frustrating Protein Skimmer

[calpoly12]

So I picked up a used 29g biocube. Everything works great except the protein skimmer. It will produce big bubbles, but not small ones. I have changed the amount of air being taken in and changed pumps. Still nothing. Anyone know what else I can try? Here is the one I have http://www.amazon.com/CPR-Bak-Pak-2-Protein-Skimmer/dp/B001EUL4XM