It could be some change in chemistry of the water, for example a surfactant has been added (stress coats, vitamins, medications, slime algae killer) all will impede skimming. Some filter medias can have the same result, Seachem Puregen and many floss and filter bags if not thoroughly rinsed. It could be the impeller is wearing out and slipping and a new 3000.600 drive unit will solve the problem, it could also be high amounts of suspended sediment in the water from a tank overhaul is causing this. In general I would trace back to when this started and note any major changes that were made to upkeep of the tank, if there were none I would change the drive unit 3000.600 and submerge the entire skimmer in a bucket deep enough to cover the entire unit, remove the air intake screw and add a gallon of vinegar and then warm water until it is entirely submerged. Plug it in and let it run for 4-6 hours so it pulls the vinegar/water mix through every passage, this should get every critical piece cleaned of salt creep and calcium build up.
