Tank your finger and run it around the lip (where the flapper makes
its seal). Are there any burrs or any high or low spots? Sometimes
toilets will have slight defects in manufacturing that will cause this
Otherwise, without seeing the job myself, there are some possible
you can take some vasoline and put a light coating on the flapper
(where it makes its seal). For a slight problem, that can help but
it's not permanant.
Also check that the chain/strap going to the handle isn't binding and
preventing a good seal. or that it isn't too tight and holding the
flapper slightly open.
If the simple shit doesn't work, you have several choices.
A. Buy a new toilet and replace the whole damn thing.
B: Buy a new tank and replace the entire tank.
C: Buy a rebuild kit and replace the fill tube (the center tube) The
kit also comes with new flappers, etc. (costs about $12.00 at Home