An older toilet's flush valve does not always (approximately one in ten flushes) does not seat after flushing. The remedy is to gently jiggle the handle so that it seats and it immediately creates a proper seal.
This was not a problem until 2 months after the valve and chain was replaced. It worked perfectly for the first 2 months. Is there a common failure-mode that causes this problem. No chlorine is used in the tank reservoir.