How does the bottom of the "plastic cup" look? If it is deformed, you will never get a good seal and will have to replace it. If it is not deformed, try using a little force to push down on the plastic cup a few times to try to get it to seal. If it still leaks, try using a new gasket until you get one that doesn't leak.
Now you see why many of us prefer to use the flapper ball type flush valve.