I have four toilets in the house (three on the second floor), and this is the only toilet that has a problem. This is one of the second-floor toilets. It is an older 5-gallon toilet.
If I flush the toilet, I get a strong water flow into the bowl, a strong swirl, and a strong whirlpool. But no siphoning. The water level in the bowl remains high throughout the flush. When the flush valve closes, the swirling stops, and the water remains at that level, instead of emptying the bowl through siphon action.
If I keep pouring buckets of hot water into the bowl, one after the other, two bucketsful siphon empty, but every third bucketful leaves water in the bowl (at the normal, neutral level). The fourth bucketful drains empty again.
Fishing with a toilet snake, I cannot find any blockage in the trapway. I have not tried removing the toilet to fish from the bottom (that is a last resort).
What are your thoughts?