Why won't my Snowthrower's wheels move when drive is " NOT Engaged "?

Machine is a yardworks, 10 hp, 28 " cut; 6 forward & 2 reverse, Friction wheel is not touching spin disk when augher is not engaged. Will move forward and reverse when drive augher is depressed but sometimes when shifting from forward to reverse it will stop moving at all. Then it will not FREE ROLL. Wheels seem to lock-up.
Yet when I drag it back to garage, place jack stands under alex spacers, and remove bottom plate, augher cable is working fine, I can reach in and move friction disk and therefor get wheels to move, But wheels will not move when I try to spin them by moving the wheels themselves. Gears are not jammed nor missing any teeth, not misaligined. I can't seem to see any mechanicial reason why wheels will not move. The wheels are on one axle with several gears attached to that alex. The friction disk is on another axle that has one gear which in turn drives the wheels when the drive augher is engaged. Would appreciate any ideas you may have