Why do FTers keep posting this stuff. OP, there is a post over in the AA forum that complains about a similar situation with AA (and, btw, that poster never has problems on UA/DL).
By the way, I've been flying since the 70's and this has ALWAYS been a problem!
Experienced flyers KNOW to check their reservations weekly and at least bi-weekly to ensure that there haven't been any schedule change,