We've had much better luck with RFC and T-Rex by calling the restaurants directly or on Open Table vs reserving through Disney. Last time we were there, the manager told us that not all their available reservations go into the Disney system, some are only available through the restaurant directly.
Do you have a Landry's card? There is a fee, which is credited to the card and you can enter in a "birthday" that coincides with your trip for another $25 reward. You get priority seating with it, too, with no ADR required. Though, for 10 it's a good idea! T-Rex, Rainforest, and Yak & Yeti table service are all Landry's owned.