If you use http://flyaow.com/class.htm you will see that their availability by class often fluctuates wildly within a matter of several minutes or hours (I'm starting to think that their yield management system uses a computer algorithm based on dice, bones, and rattles). Flights may go from no availabilty to some seats open to wide open and back multiple times, up until the flight does truly sell out (at which point it seems to stabilize). I recently experienced a lot of frustration trying to book tickets for myself and friend on 2 different reservations (different departure cities) because of this.

Moral of the story: If the flight you want has no availability in the class you want, don't give up. Check back and it's very possible that a seat will eventually open up.