I just replaced the old flap (that was about 2-3 years old) with the exact same brand...and it still leaks; tank refills at least 3-4 times an hour. if I manually press on the flap it seals tightly..until the toilet is flushed again, when it doesn't seal as tightly on its own.
The tank has a brass pipe and fitting; wondering if I need to sandpaper or gently file the brass ring into which the flap fits to create a tight seal. I believe I had to do this once before when I replaced the flap, but I don't remember what I may have used. We have a water softener system so it is NOT hardwater build-up.