Toilet not always flushing fully, what could be wrong?
I will describe the test you need to do to check the toilet.


Remove the tank cover, be sure the tank is full of water to the full mark.
Push the flush handle.
When the rubber flapper flips up, reach down and hold it in the up position until all the water has drained from the tank.


If the toilet flushes completely under this test, but not on it's own, then your flapper is closing too soon, or the tank is not filling as much as it needs to do.  The flapper is a real easy replacement, if the tank is not filling, then the fill valve needs to be replaced.

If it did not flush completely, time to spring and spend 200 bucks for a new Class 5 toilet.  Which toilet to buy is a whole new topic.  

Good luck.