That's the first thing I'd check..
If it happens in both the wet and dry mode, it's most likely a blocked tank vent. However, if it ONLY happens in the dry mode, it's more likely to be a failed wet/dry cam...a VERY common problem in Jabsco toilets made in the last 10 years. Try jiggling the lever to see if that helps...if it does, that's definitely the problem. Call Jabsco customer service for a replacement part...or make this just the excuse you need to upgrade to a PHC/PH II. The "conversion" costs very little more than a complete new Jabsco. Check it out:
sbo.com PHII PHC LBA
If it does turn out to be the wet/dry cam, that doesn't excuse you from checking the tank vent. In fact, that you even had to ask tells me you aren't maintaining it. Unless you want major problems, you should be checking the tank vent regularly (also all the hose connections and clamps in the whole system!) and backflush it every time you pump out and/or wash the boat.
