What restaurant are you trying to book and what site are you using to try to book? I just had a look at a few and it looks like reservations are open through the end of November for most and into December for a few.
Its funny - looks like, for the theme park restaurants, Lombards can be booked through the end of Nov but the other three (Finnegans, Mythos & Confisco) are only open through end of October. There doesn't seem to be any consistency, unfortunately.