I too have a 15~91. Make sure there is no fuzz or threads in the bobbin case. I just blow it clean or use a make up brush when i change bobbin threads. Also rethread it again from the start. It may look right buta little off. It is sewing fine for a bit so looks like may just be dirt or thread off. Make sure the needle is threaded right. Mine is right to left and the bobbin thread is in correctly. I love mine. Its about all i use anymore!!!