I have a sneaking suspicion that you may be the victim of technology to which there is no easy solution.

When a drain line is properly sized under full load the level of liquid in a horizontal section of the pipe should fill the pipe ½ full. This allows a sufficient depth of liquid to suspend and convey any solids that might be present in the waste while still leaving the upper ½ of the pipe open for the free transfer of vent air.

Back in the day of the 3.5gpf toilets it was determined that all toilets must be connected to a 4" drain line.

When they first introduced the 1.6gpf water saver toilets there was a great number of complaints with the same symptoms as you describe, and as a result the toilets got a bad rap, however it was soon determined that if the waste line was reduced from 4" to 3" the problem would be corrected.

The codes now require that all toilets with a discharge greater than 1.6gpf must be connected to a 4" line while all toilets with a 1.6gpf or less may be installed on a 4" line during a retrofit but for new construction they must be connected to a 3" line.

In a typical single family dwelling changing the line may be costly but it is not out of the question however in the case of a condo changing the line would be a major project.

Hi,I at one time had to maintain about 1000 Kholer toilets 1.6 gal type. I had the same problem. I found the flapper had a float and when I lowered the float more water went into the bowl and the problem was cured. Maybe if your toilet doesn't have a float maybe you could hook a fishing bobber on the flapper chain to get more water in the line.
I know, I know, what about the water, well untill our congressman helps repair flooded ceilings and floors and pay for damages we just have to use what ever we have.
Good Luck Woodbutcher

Yep, I have the same problem. My plumber said the flush isn't powerful enough and it allows for waste to clog the line. Wish i had never installed it, my old regular flush toilet worked great never had a problem. Now I live with a plunger next to my Duravit toilet!