If your toilet worked well in the past and you live in an area with hard water, chances are the rinse holes around the bottom of the rim have become clogged with lime deposits. Clear rinse and siphon holes are crucial for complete flushing action. Even though the water from the tank will eventually find its way into the bowl, high water volume on the first surge is important for good flushing. There has to be a “critical mass” of water for solids to be flushed.
As a first step, ream out the rinse holes with a bent coat hanger (Photo A). To do a thorough job, dry the bottom of the rim, then roll up paper towel “ropes” and seal them against the bottom of the rinse holes with plumber's putty pushed against the bottom of the rim (Photo B). Then seal the siphon jet hole with another glob of putty and pour a bottle of lime remover into the overflow pipe (Photo C). Let it sit for at least eight hours to allow the lime remover to dissolve deposits. Remove everything and flush the toilet several times.