Old thread, but likely same problem:
I found it was my travel agent had a slightly different combination of first name + middle name/initial + last name than my United MP profile.
Agent fixed my Sabre profile, matter fix.
Though if you are a SELPAX/SSSS -- which seems to happen more often on flights INTO the US than within or out, it may still happen.
It bites when you are trying to score a CPU based on your check-in time.