Snow Blowers

I have a snow thrower model 10527-ste. Yesterday, when I used the machine, when I released the traction drive control lever, the transmission did not disengage, the machine kept moving forward. I wrestled with it to keep blowing snow and after a few minutes it disengaged. Then a while later the same thing happened. This time it did not disengage and it is locked in gear. Is there some adjustment I can make? I looked in the service manual but there is nothing about this kind of problem. Help!
Thanks, JA