[NYCAP123]

This early in the season you may also be dealing with the dreaded "Brick". If you don't pump out often or completely enough the waste can settle into a brick in the bottom of the tank. Since 1989 that could be a big brick showing you a full tank, but no lose stuff. Each time you pump out you should put in several gallons of fresh water and then pump that out. Personally I don't know of any chemicals that can undo a brick without hurting the tank or "pipes". Maybe someone else here does. In the mean time we'll just hope it's the vent.
One spot of possible good news. More often than not when I've seen boats unable to get pumped out it's the pump that doesn't have enough suction. Try another pumpout station before getting too dirty.