Peter,
On the clutch shaft you have a collar that adjust the spring pressure on the clutch(on left hand side when you are behind the mower). Try releaving that so that you can get a bit more play. I think that you have about 3-4 positions for the tension.
Also make sure that your chain is not to tight that goes from the clutch to the cutting cylinder as that will cause you problems as well when pulling the machine in a backwards direction(you need about 1/2"play on chain).
