Cub Cadet Lawn Tractor will not restart

I have a 1 year old Cub Cadet lawn tractor that will start and stay running if I start it by priming the carburetor with a little gas. Fuel filter and fuel pump seem to be working. When I stop the tractor, basically it automatically turns the engine off when I get off the seat, it will not restart. The only way to get it running again is to do the same - put a little gas in the carburetor. What else should I check?