You may have a crack in the tank allowing the water out.
Or the seal on the douglas valve may be leaking but these are rare (The douglas valve holds the flapper)
When you replaced the flapper did you use a Korky brand?
Check this, Turn the water to the toilet off and drain the tank down. Remove the flapper, and run your finger, lightly, around the edge that the flapper would seal on if it was in place. You are feeling for anything but a perfectly smooth surface. There could be hard water deposits built up on it not allowing it to seal right.
If there is, try using your fingernail to break it off. Don't use a knife or similar as it could gouge the surface and ruin the surface making things worse.