Some locations (franchise... ILM in particular) still refuse upgrades claiming "that's all I have" no matter what Avis.com or their lot show as available. They started with "I can only do a 1-class upgrade" until I called the CC desk from their location once. Now they give the "none available excuse." I often reserve full-size if I have to rent from there so I'm guaranteed better than a Corolla. Or, I'll just drive to RDU (2 hrs away) to prevent having to give them my money.

In any case, it could be something like the above reason for booking full. Also, during busy times, it is worth booking larger cars. It's possible you get there and they honestly don't have any upgrades available. I'd rather the $2 or so a day for a better car than intermediate.