Yamaha banshee won't rev in neutral?

i have an 03 banshee that wont rev in neutral. i started it the other day and let it warm up then went to cracked the throttle and she did not rev. then i tried putting it i gear and the thing worked like a charm. the bike was in the back of my truck and had the brake on when i started it. i was thinking that could be it but i started it up a few days before that with it on and she worked fine.