There is a mega-thread that addresses this many times over:
But to save some digging:
Have you checked in already when you check the airport list?
Is your MP number correct on the reservation?
Did you purchase the ticket before 3/3/2012?
Which list is your travel companion, the 1)upgrade cleared or 2)upgrade standby?
And, by the way, UA has indicated that the "pending" feature will probably return some time this year.