Rub your finger around the seal where the flapper goes...if it is rough or has a chip or lump on it, the flapper can't seal properly. Some flappers have both ears and a ring...if yours do, do NOT use both, cut the ring off so only the ears are holding the flapper...using both can often distort the thing, warping it and preventing it from sealing.
Last thing, the gasket that is holding the flush valve assembly is held together with a big nut on the bottom of the tank. If that is loose or the gasket degraded, it can leak. You may just need to remove the tank and tighten the nut holding things in place.