Could be several reasons
The vent line only SEEMS clear??? Be sure. And be sure that the vent fitting on the hull is also clear. If they are, look for an obstruction or a kink somewhere in the head discharge line. Something is creating backpressure. I'd also recommend that you take this as a sign that it's time--PAST time--to rebuild the toilet....'cuz it wouldn't be able to belch--instead, you'd feel the backpressure--if the joker valve weren't worn past preventing backflow from backpressure...and if it's worn, so are all the other seals etc. So you really have two projects: find the obstruction and clear it...and rebuild (or replace, depending upon age and price) the toilet.
