We have one toilet that will not flush well. It is not stopped up. We tried the test were you poor a 5 gallon bucket of water in as fast as you can, and it never over flowed. We have tried cleaning all of the little water ports around the rim of the bowl. We have run a snake up in it several times also, but nothing. The other toilet is working fine, so we do not think it is a draingae problem.
Not knowing the type of toilet you have, 1.6 , 3.5 , 5 gallons/flush I can only advise to try these options. See if your toilet has a jet(hole) in the bottom of the bowl, If it does make sure this open. Make sure your ballcock is letting the proper amount of water flow (not taking any longer to fill the bowl, you can compare with your other toilet,some toilets are dependent on this for a proper flush. I have had problems with older mansfield toilets in this area , not saying mansfield is a bad product, but they are in my area prone to these problems, as they do not have jets in the bottom of the bowls on the 3.5 gallon flush models, the 1.6 gallon models do have the jets @ I have not experienced the problems with them.