How much of the skimmer is submerged in your sump?
I have a 3000 and at first was submerged 8", then moved it up a little to be at 6".
Secondly how high do you have your bubbles going up the neck?
I recently posted a similar question on RC and was advised to increase the bubbles to just about an inch before overflowing into the collection cup. It is a fine line between skimming wet and skimming dry to get a nice dark green sludge. Any bit of aerosol (Febreeze near tank,hairspray etc), food particles, additives, water change or hands in tank can set a skimmer into crazy mode as in overflowing or not producing at all.
What did you clean it with?
If just diluted vinegar can take some time (week or so) to build up a good slime coat again. If anything else (Dawn,Joy etc) may take longer.