What should I do about my toilet flapper sticking open?
If you purchased one of the generic replacements, they usually come with both the slide on and hinge mounting rubber parts attached.  If your mounting is the two side pin type, then you must cut the connecting tabs of rubber that hold the slide on rubber ring, and discard it. If you slide the flapper on with the ring and also use the side hinges, it will not work properly.  You can only use one mount type..  slide on or side hinge.   Hope this is your situation, easy fix.