Most likely a blocked tank vent
If air inside the tank displaced by incoming flush can't get out the vent, the system beccomes pressurized, creating backpressure that makes the toilet increasingly hard to pump.
The most common locations for a vent blockage are the vent thru-hull and the connection to the tank--both the tank fitting and that end of the vent line. Check the thru-hull first...scrape out anything you find in it. If there's a screen, knock it out...it's not serving any useful purpose. Before removing the vent line from the tank, relieve any pressure in the system by opening the deck pumpout cap VERRRY carefully...'cuz if the system is pressurized, waste will erupt.
However-- If the toilet pumps easily in the DRY mode, is only hard to pump in the wet mode, there's another possibility: a failed wet/dry cam. It's a VERY common problem in Jabsco toilets made in the last 8-9 years. The cam hangs, creating what FEELS like backpressure from a clog. Jiggling the lever may free it up temporarily, but the only real cure is a replacement cam assembly.
