Reciently one of our second floor toilets has begun losing water from the bowl. When it is flushed, it fills to the proper level and shortly after refilling the tank, all is silent. There is no apparent leakage and the water level may stay up for quite some time. However, it seems that after the first floor toilet is flushed several times, the water in this second floor toilet bowl will be low. The other second floor toilet is unaffected and the water in the bowl losing water never gets low enough to expose the trap, so, we don't have an issue with sewer gas backing up into the room.

It seemed to me that this might be due to a blockage in the vent pipe. So, I went up on the roof and ran the hose down the vent full blast. The water ran freely with no sign of backing up even after several minutes of running, yet, the toilet still exhibits this problem of losing water.

Any suggestions on how to fix this?