Sounds like you have problem that's very common in
Jabsco manual toilets--a failed wet/dry cam. The wet/dry "valve" is actually just a little "gate" (the cam) that the lever swings to block or unblock the flow of incoming flush
water. Due to either a tooling or mfr'g flaw, it hangs up...creating back pressure that feels like pumping against a clog or blocked tank vent, except it only happens in one mode--usually, but not always, the wet mode.
If this is the problem, jiggling the wet/dry control sometimes relieves the problem temporarily.
The cure: replace the wet/dry cam assembly.
Jabsco often sends out a new one for no charge...but then you have to take the pump apart to replace it, which can be a PITA and really messy because you can't flush
water through the pump to clean it out before you take it apart.
So my advice: just replace the pump instead....modern Jabsco toilets have a fairly short lifespan--they're purposely made to be "disposable" like everything else today. A
service kit--which doesn't include the wet/dry cam assembly-- costs almost as much as a pump, which doesn't cost that much anyway...and swapping it out only requires removing and replacing the 4 bolts that hold the pump on the base and two hoses, which beats the socks off taking the pump apart!
I know a bunch of Jabsco manual toilet owners, including one longtime member here, who just replace the pump every spring as "preventive maintenance."
--Peggie