I thought what Jason meant was the roller (bearing) behind the blade in the guide head each side of the cutting area of the blade. If this is set too far forward it will always push the blade off the wheel no matter how the tracking is adjusted.
By "guide head" I mean the part with three bearings, two just touch the sides of the blade to keep it upright, the third is behind and stops the blade being pushed back. They are all adjustable. Loosen the one at the back in each head, then push the blade fully onto the drive wheel, the teeth should be just over the front edge, then adjust the back rollers to just kiss the blade.
I can't remember what the makers call this part, so hope you know what I mean.
Phil