Zig Zag Stitch Not Working on Veritas 8014/26?

I have a Veritas 8014/26. It was the first sewing machine my mother ever got, so I love it really much! It seems that the "zig zag" feature has a problem. When you choose the zig zag stitching, the needle does not seem to cover the right distance (from left to right and backwards). Where should I look to find the problem?