I have two old Singer machines, almost exactly alike. The one I am presently using, up north, was giving me trouble, the thread constantly breaking, when I would change color I found that it was working fine, this continued back and forth until I realized that when the thread was breaking it was because the bobbin was not wound properly. It took me days to figure that out. I use metal and have at least three different type of plastics and they all work in both my machines (from the '60s) if properly wound. Just something to check. I purchased a set of 6 prewound bobbins and cannot get the thread loose on any of them to be able to use them.