DerbyDad03 wrote:
....
I checked the plastic rod from the handle - it's straight and tight -
and the handle isn't tilted in any way. As far as I can tell nothing
has changed in the many years since the guts were installed, yet for
some reason I had to shorten the chain to get the flap to stay up.
....

Rubber in the flapper stretched/deformed w/ time so in effect the chain
had become longer.

--