Could you post what water tests you have done so far and what your readings are? Assuming depth is correct and everything is mechanically sound on the skimmer, try cleaning out the collection cup, re installing it and then cranking up the flow on the skimmer so you are intentionally over flowing it, take out the stopper and let it flow back to the sump. Leave it like this for an hour or so and see if this helps. Its worked for me in the past.