Not every seat on the plane is available for pre-assignment. (And recall that overbooked conditions mean more people than seats, so even if all seats could be assigned some people would be denied.) Delta may hold back something like 15 seats on an A319 for assignment at the gate. Check again for possible openings (as elites get upgraded from 5 days down to 3 hours in advance of scheduled departure) but don't worry about it too much.