In addition to Dodgemans comment, sometimes the flapper is just the wrong one for the situation. Many of the new flappers are rated for 1.5 gpm, and they do not work properly in an old toilet. Some of the flappers with a rubber chain are pushed down when the handle falls back into place. And some of the flappers have too large of a hole in the float, so they fall sooner than they should. Another problem is the flappers are not flipping all the way up, because the ears on the flush valve are too tight for the holes on the flapper. When this happens, the flapper is only flexing on the arms, rather than flipping up on the ears. Since the arms are rubber, its like a spring pushing down on the flapper. Also make sure you break off the connecting piece on the back side of the flapper that is used on ears on the overflow tube. Flappers come with an attaching piece between the ear holes. This is because manufacturers make these flappers universal for multiple brands of toilets, and the connecting piece is for a different brand of flush valve.