Many people wish they had your problem! Your skimmer is doing its job very well without and excess of water and drop in salinity from wet skimming. You must have a pretty high organics content and my guess is the skimmate will taper off as the organics are under control.
What are the particulars on your system such as size, bioload, feeding habits etc? Is there a way can reduce the organics present with things like more ferquent of larger water changes, reducing feeding or a lower bioload?