Product review (1 stars): Poor quality-buy Heartland instead
I bought two of these.  The 'cup-like' flapper device in one is not round so it sticks.  In the other while more round, it too is flexible and not fully round.  Neither one always come down to completely seal.  General poor fit and finish.

I just bought a replacement from Heartland which has a virtually identical design.  It seems to be a far higher quality plastic with much better fit and function.  It opens and seals easily and completely.