How can I fix Kenwood Chef - the drive cog mechanism in the gearbox?

I have a Kenwood chef A703C (Australia). The drive mechanism in the gearbox will not activate because the main (plastic black ) belt driven cog in the gearbox (that is driven by the motor) won't pull down to engage the mixer arm ("K whisk"). The spring seems OK - I am at a loss as to why it won't pull down and engage - any help would be greatly appreciated.
Bob