I have had a Toto Ultramax for six years--low-flow, of course, I'm a Californian.
It's in a guest bathroom and, now that my house guest has been here for three weeks, she mentioned that the toilet bowl doesn't clear all the way and that it has gotten worse over time.
Sure enough, when flushed, the bowl now fills all the way but doesn't go down. (OK, left to sit for hours, the water eventually goes down the drain.)
WHAT I HAVE DONE SO FAR:
1. Correctly used a heavy-duty plunger with force, several times in a row, and on several occasions.
RESULT: No difference
2. Used a hand-crank snake about 4-6-feet long.
RESULT: No clog found, no difference in water level.
NOW I'm stumped, because these two methods have always worked in the past.
All other plumbing in the house works/drains fine. The bathtub next to the toilet works fine, so I think this means that the sewer line to the street is OK. This bathtub and toilet are at the nearest point to the front of the house and sewer line to the street. I should add that the (solid) sewer line was replaced ten years ago, as the roots of a palm tree overhead would block the line about yearly and I'd call Roto-Rooter. (The palm grew very VERY tall thanks to that great nutrition.)
I have a regular handyman coming tomorrow (or soon) who is not a plumber. All he can suggest is to remove the toilet, use a longer snake, and maybe there is a blockage further along the line. But as I wrote above, there is no specific evidence of this.