I have a Craftsman Eager 1 Hi-Torque power 6.5hp lawnmower that does not self-propel when engaged as the engine is running, but, when the engine is not running, I'll lock up the front wheels by and the wheels feel like they're engaged when I try to spin them. What would cause the wheels not to spin when the engine is running and the lever is in the lock position to self-propel?