Hi Mark,
There are several things to check:-
The tension dial on the yarn brake - make sure the spring is not pulling up too high, if it is tighten the tension slightly.
The brushes under the sinker plate - are they turning freely and do they have any bristles sticking out at the wrong angle? Trim rogue bristles with sharp scissors. If the wheels are stiff they may have yarn caught under them, remove and replace one at a time and clean the spindle. Clear away any build up of fibres in the brushes.
Don't take the carriage too far past the knitting - just as far as you hear a 'click'.
Hope this helps,
Sue, July 2011