Sort of a follow-up on Darin's comments above.
Assuming you did not change anything (or anything was damaged) sinve it was collecting considerable wastes and since you feel it is clean then it is likely that you may have removed the majority of skimmable DOC
. This is not uncommon when a skimmer is larger than required for the bio-load of the tank it is skimming.
Assuming that you see a lot of bubbles in the reaction chamber - it is a good guess that it is working fine but there just aren't enough disolved organic compounds to be skimmed.