Is the flapper closing to soon? If so not enough water is flowing into the tank to make it flush properly would be my first guess If it is working as it should what happens If you dump a four gallon bucket of water directly into the tank at one time? Does the water go down as it should or does it work a bit better?

IF not there can be only two things else that would be wrong. One is the vent to the toilet might be plugged. IF there is another toilet located above this one, they may share the same pipe which works both as a vent and drain. So check that first, even if there is nothing above, check your vent on the roof for blockage. A blocked vent often prevents drain water both in sinks and toilets from draining as fast as it should. Sort of a suction builds up causing the water in the toilet to glub glub as it is trying to get air to allow it to drain down past the toilet into the main drain.

The 2nd thing that can cause this is something further down the drain that the snake has not got to. If the main drain is partly blocked air pressure builds up within the drain as the large amount of water that is flushed at one time pushes down the pipe. The result is sort of a push back because of the air pressure that builds up with in the pipe. You may need to have a professional come in and use a camera in the drain line to see if that is the case. This partly blocked pipe can be just settlement of the main line filling with water and acting like a large trap. If you have a newer toilet with low gallon flush, there is not enough water leaving the toilet to push against a larger quantity of water sitting inside the drain pipe past the house but before the road connection.

Midlantic Contracting LLC...