My Bernina 830 won't zig zag any more?

Hi,
The stitch width lever on my machine won't move the needle to right or left - well only very slightly, certainly not as far as it should or as far as it used to. The needle positioning knob does still work and will keep the needle to the left, centre or right. And yes, I've tried moving the lever on top of the machine to the right position ie. not for the decorative or utility stitches, but still nothing I do causes the stitch width to engage.
This used to happen occasionally and I could temporarily 'fix' it by moving the top lever up to the top and then back to it's proper zig zag position, this obviously 'jolted' it back into position and it would work properly but now I can never do zig-zag.
Would be grateful for any ideas of how to fix this.