not so easy to give proper answer without seeing precisely what it does but few suggestions..make sure take up spring is working,ensure tension is set correctly,set position finger cap little bit bigger,make sure timing /hook throw is correct(that mean the hook must go back far enought to "throw"cotton when passing over bobbin case.if you checked all this the changes are good the problem will be solved.if not suggest you take it to good repair shop for inspection
johan, May 2012
