Over the past few days one of my toilets had been giving me a bit of
trouble.

Normally I could just push the handle down and let go. The flap would
stay up until the tank was empty and I would get a complete flush.

Recently I found that the flap was dropping down too soon (almost
immediately) so the toilet wouldn't flush. I had to hold the handle
down for a little while so the flap would stay up and give me a full
flush - not the whole time it took to empty the tank, but longer than
just a single "push and let go."

This morning I shortened the plastic chain by a couple of links and
it's back to flushing with a quick push of the handle.

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.

Any thoughts?