It can be one of two things.
The flapper could be leaking through into the bowl. Even though you have changed the flapper, it might not be sealing against the rim of the flush valve. There could be buildup on the rim, the rim might have imperfections on it, the flapper could be slightly offsize and not seal against the flush rim.
The rubber gasket that seals the flush valve against the tank could be leaking, causing the same symptom as a leaking flapper.
Turn off the water supply to the toilet, and push the flush handle down and hold it down for 30 seconds or so. Then take a sponge and soak up just a little water out of the bottom of the tank. Then add food coloring to the water that is left in the tank. The idea is that the water level is just a tiny bit below the flapper, so it won't leak through the flapper.
If after several minutes there is food coloring present in the bowl, it is the seal between the tank and the bottom of the flush valve.
If there is no food coloring present in the bowl after several minutes, add a little water to the tank, so that the flapper is covered. Let it stand again, if the food coloring shows in the bowl, then the flapper is not sealing for some reason.
