I have a problem with the flapper in my toilet tank. I have replaced the flapper twice within the last 6 months, the last time about one month ago. There is no indication of flapper deterioration.

The problem that I am having is that the bulb does not always seat properly in the valve opening and the water
runs until I reseat it. The flapper does not seem to drop properly and catches on one side of the base ears. The instructions on the replacement package say to "Adjust so that bulb is centered over valve opening". Although this might be the problem, I see no way of adjusting it.
Do you have any suggestions?

Thanks,
Alan