They're blocked for mobility accommodations (handled by phone rez usually) until DoD when the GA can assign them.

I busted my knees a bit back and had some trouble walking, and a call to DL was able to get the first C+ aisle seat assigned to me on all of my upcoming reservations, with the warning that anyone with a fused leg, or some other mobility issues could get me booted from that seat (if their legs were more screwed up than mine, I had no problem letting have it).