Your problem maybe due to a clogged flush rim.
Try using a 5 gal. bucket to test this. Fill the bucket with around 3 gal. of water, dump into bowl if it flushes OK then your problem is the flush rim is clogged with hard water scale. You maybe able to save your toilet by using muatic acid which you can get in any home center. This is the same kind one uses for swimming pool cleaning. To treat take off the tank lid and pour about 1/2 gal down overflow tube (this is the tube in the center of tank) becarefull not to splash this is a weak acid but still could cause burns and do not inhale vapers.let set for a couple of min. flush toilet it should flush better.