I replaced everything in my brothers stool not too long ago and I last but not least, I tore the back of the stool off and replaced the gaskets between the stool and tank. Sometimes they go bad and this is what the flapper sits on. Not to hard to do, but be careful and do not over tighten the bolts, may crack tank. Other than that, if it is a cheap stool, sometimes they have flaws in them and cannot be fixed. If continues, sometimes easier to replace the stool. That is not hard to do either.
Yes his stool quit leaking down and flushing..
Gene Stewart