Tank vent is the first thing to check...
If it's open, the other most likely source of your problem is a failed wet/dry valve--which isn't actually a valve, but a little gate (Jabsco calls it a cam) that the lever swings to block and unblock the flow of incoming flush water. Due to either a tooling or mfr'g defect, that little gate has a nasty tendency to hang, creating a feeling of backpressure that many owners mistake for a clog. Greasing the toilet prob'ly un-hung it briefly. This is a VERY common problem in Jabsco toilets made in the last 7-8 years...one that Jabsco doesn't seem inclined to fix...at least not until the mold in which that little gate is made wears out...if then.Do yourself a HUGE favor: bite the bullet and replace that hunk of junk with a Raritan PH II...or PHC, which is the same pump on a compact base, if the full size PH II won't fit in your head.