T-Rex is popular, but not that small, and doesn't usually book up 6 mos. in advance unless you're going during a prime holiday week.
We had no trouble booking for a lunch last week, and I even changed the time twice in the last month before our arrival for a Friday lunch (5/11)
I thought maybe they hadn't "loaded" T-Rex for November 2012 yet, but just tried party of 4 for lunch on 11/14...and everything at Downtown Disney, including TRex, was available between 11:30 & 12:30.
What day are you trying for, and what party size/time? If you have an large party, it may be more difficult...