Normally the belt should be slipping around the drum if the pilot is disengaged,sometimes the belt does not slip smooth due to the conditio of the belt or dirt,grease or oil on the drum inside ,take the unit apart ,check for belt condition,exessive belt tension ,if necessary belt tension can be adjusted on the rear of the backplate.

Mine did the same thing yesterday - I had left the clutch lever engaged when I last left the boat and the belt had adhered itself to the drum. The belt then skips over the drive cog as you turn the wheel instead of the belt slipping round the drum. I gently turned the wheel lock to lock and the belt unattached itself from the drum and all was well. In future I will not use the clutch as a wheel lock when the boat is in its pen and will also perhaps release the clutch from time to time when using the pilot.