There are lots of reasons a flapper will not seal properly.
The flush valve seat that it rests on might be pitted or dirty.
Very common problem.
The flush valve is leaking at the bottom of the tank, around its gasket, and the flapper is not actually the problem.
You have the wrong shape or style of flapper for that flush valve.
The flapper got deformed while sitting around on the shelf, so it is not laying like it should be, perfectly in contact against the flush valve seat.
The flush lever chain is too tight, the clip at the top of the chain is getting hung up, or you are hanging the chain from the wrong mounting hole on the flush lever, so it is getting pulled off-center.