I have the same problem, but only on the toilet in the basement. The toilet is new - 6.5 litres. All the plumbing was inspected. I have a feeling this has to do with poor workmanship of the plumber who positioned the pipe before the concrete was poured (the same plumber terminated a sink vent pipe in an upstairs wall by gluing a temporary orange cover on it, had the vent pipe for the kitchen sink sloped down instead of up, and didn't place a temporary cap on the roughed in - no trap -shower drain in the basement!). Initially, I installed a 13 litre flush toilet which worked fine, but the inspector (rightly) had me replace it with a 6.5 litre flush toilet. If I use it immediately after the tank has refilled, it will flush properly, but if I wait for 30 seconds, it just swirls around until the flapper closes, but does not drain. I suspect the drain pipe under the floor is incorrectly angled or there is a vent problem. The 6.5 litre toilet on the main floor works fine. Also, the contractor told me there was some kind of valve on the sewer line to keep sewer gas from entering the house if the traps dry up (?? ). Any ideas?

You piggybacked on a old thread so I gave you your very own.
You might consider this.
In my capacity as shop trouble shooter I began to run into a slew of complaints on our new construction that were just like yours. I knew they all couldn't be " bad toilets" so I went looking for the cause. I found that when installing the new bowl that the wax ring had been forced out into the discharge pipe and blocking the free flow of water. When the water hit it a backpressure was set up preventing the siphon that makes a good flush and the solids just swirled around while the water went slowly down. I had to set up meeting with our plumbers and show them the problem and how to fix it. When you set the bowl, put the wax seal down on the flat side to the closet flange. Then take your hands and bevel the wax seal outward so that when you set a bowl on it the wax is forced out instead of in blocking the flush. The customer complaints stopped and I had less work to do. Try molding the wax seal so it doesn't choke down on the flush. Good luck, Tom