I'm new to the Delta Elite program, so this may be a newbie issue. Yesterday I booked two round trip flights and after booking, I checked the "Upgrade Request" button. For one of the round trips, the upgrade request status turned into a green check mark with "Upgrade Requested" stated. On the other flight, it said "Unable to Process at this time" in red and didn't change the upgrade request status.

Is there something else I should be looking at or checking?