First thing is to feel around the seat where the flapper mates - it must be smooth with no cracks, chips, bumps, etc. If it is not smooth, if you're lucky, you can just clean it carefully. Then, verify that the chain length is proper, and the flapper can go down all the way, maybe one or two links slack. If the flapper is being pulled up towards the side, sometimes, that can prevent it from seating. Last (I can think of, anyway) is that the things are designed to either use the ears to hold it in place OR the ring - you CANNOT use both! If it has ears and a ring, cut the ring out!.
Well, maybe one more thing, this normally only occurs when the thing is new, the gasket that holds the flapper seal in place may be leaking...typically, there's a big nut on the bottom of the tank, tightening that may resolve the issue.