It could be just the run in period of the skimmer..What i ahve always suggested is to strip the skimmer down and wash it in warm soapy water, all over and rinse complelely in running cold water and assemble and re-fit..When skimmers are made, well plastic products as a whole, have a film all over the plastic, as protection, and this will reduce the effectiveness of teh skimmer....
I ran mine in a solution of water and white vinegar for a couple of hours and then ran regular water though it to clean it out. It ran it for 3 days with very little skim. I then took it (after 3 days) and cleaned out the glass tower (water and paper towel) and the next day woke up with about 2 inches of skim. Now it skims great.
Run it in a bucket and then add the vinegar - it should start producing big time bubbles. I have read that the glass tower stem needs to be clean.
Apparently putting your arm in the tank a lot can also prevent the formation (and stabilisation) of bubbles, something to do with the oils on the skin going into the water and destabilising the bubbles.
NB: I've never noticed this myself but the info is from an experienced reef keeper.