If your machine has a dial to adjust the stitch width, then make sure you are also turning this to the right setting so the needle "swings" as part of the stitch formation. However, if you are certain you have it set correctly, then other possibility is the needle bar bearings have siezed up or there is a spring inside the same area that pulls the needle back to centre, maybe its broken or come off. Either way you need to get inside the find out the answer; if you are not experienced in pulling apart a sewing machine, then I'd suggest its time for a trip to the SM technician for a service.
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If your machine has a dial to adjust the stitch width, then make sure you are also turning this to the right setting so the needle "swings" as part of the stitch formation. However, if you are certain you have it set correctly, then other possibility is the needle bar bearings have siezed up or there is a spring inside the same area that pulls the needle back to centre, maybe its broken or come off. Either way you need to get inside the find out the answer; if you are not experienced in pulling apart a sewing machine, then I'd suggest its time for a trip to the SM technician for a service.