"G. Ross" wrote in
:

Had the same problem. Took my old flapper to the local ACE and was
glad I did. They had about 6 different styles, some with shorter arms
to the hinge, some longer. I had previously discovered that if the
cone doesn't sit right in the hole it doesn't fit right.

It's actually worse than that - not only do you need the right
length so the cone seals on the seat, but flappers come in "new"
and "old" versions that look identical. The "new" kind are for
1994 and newer toilets that flush with less water, if you put
one of those on an older toilet it won't flush fully.

John