Have you checked the clutch shoes? If they are worn, they can't grab the bell pinion and engage the drive train.

This may help, check page 45 of this manual download.

[link=http://media.hobbypeople.net/manual/145621.pdf]XTM MMT Manual[/link]

One thing for tip, on most nitro's clutch shoes fan out in the spinning direction. On the big XTM MMT, and Mammoths they go on opposite of the spin.