One of our toilets short-flushes. That is, unless you hold the handle down (and thus the flapper up) for an extra couple of seconds, the flapper closes too soon and doesn't release the entire tank of water, instead of floating down with the water level. I've adjusted the pull chain so that there's only a little bit of slack at rest. No joy. Time for a new flapper, or what else can I try? TIA!