I agree wioth the above posters, as the OP flight is UAX they should watch the FLIFO screens very closely. Also might be worth a trip to the gate area just to check on the situation.

The main problem when UAX gets delayed is that is much easier to tail swap, load the flight, and close the doors than with mainline UA. Perspective here is that with 76 seats and lots of little RJ hanging around then changes can happen in matter of minutes. Comparison is that airlines can't turn a 747 in under one hour and tail swapping a mainline narrowbody flight is a lot more complicated.