A toilet that flushes poorly, or empties too slowly or not completely is maddening. The hope is that it’s some kind of small blockage. The way to find out is to break out the toilet plunger. Do not be shy, plunge it hard for best results.
If this is not effective then the next thing to check is water level in the toilet tank. There is a fill line in the tank and you want to make sure the water level is within ¼” of this line. Simply take the tank lid off to check the water level.
If the water level is too low then that will explain the poor flushing performance. If this is the case make sure that the water coming out of the fill tube, which fills the tank, is not being directed into the toilet bowl pipe instead. Flush the toilet and observe.
If these solutions do not solve the flushing problem then it’s probably time to call in your licensed plumber. There are likely venting issues and he is best equipped to deal with these issues.