Singer 2250: zig zag stitch prob

Hi friends, I have a singer tradition 2250 model sewing machine. This has stopped sewing zig zag stitches, it stitches in plain fashion instead of zig zag. I can clearly see the needle moving to the left and right and penetrating the fabric, but the needle again pulls back the thread from the fabric while zig zag. What could be the issue, have done every thing by setting up the tension and replacing different needles, now I doubt if the shuttle hook is to be replaced? please help.