Thread catches
I have a singer 401A, 431G, and a 503. I can sometimes sew for quite a while with them and sometimes for only an inch or less and then the top thread seems to “ catch” and bind up, making a knot. I have to stop, pull it out, and start sewing again. This happens with all of these machines. Does anyone have an idea of what might be causing this? DH and I are just getting started on working on our machines. We have done the oiling and have taken the bobbin case out to clean and make sure there are no leftover threads. We don’t know what to try next. Thanks in advance for any help.