Not all flapper valves are created equal...some of them end up getting to you slightly warped. Somewhat depends on the packaging and the quality of the things. The new one could still be leaking. Quick way to tell, use some food coloring in the tank, see if any leaks into the bowl. Sometimes the seat has deposits on it...rub you finger around it and make sure it is clean and smooth. The gasket around the flapper hole could be bad, but it might leak onto the floor rather than through to the bowl.