The awards that require a lot of points will buy the last seat on the plane so are the equivalent of full fare. The cheaper awards are tightly controlled and are snapped up on a popular routes almost a year ahead when they are released. After that it is a matter of luck. I had an award for MSN-PHL-MAN last year that I had to cancel, I was able to apply it this month to flights in July and then to change to September. That was surprising but I suspect it is a route that AA is trying to fill! Note that changes are free if you keep the same start and end so you can pick a bad routing if one is available and hope to be able to change.