I have made over 60 face masks on my old Singer 604e sewing…

You will remove your needle -plate, bobbin and bobbin case and clean that area out completely of any excess thread, lint or debris. You can do this with your brush that was provided, or simply use a tweezer. Once that is done, place a drop of sewing machine oil in the center of your shuttle ( metal piece holding bobbin and bobbin case ) Re assemble your bobbin area. Now, moving to the top. Un thread your machine and raise your presser foot so that your tension discs are open. Take a scrap piece of material and run it through your top tension back and forth in the pathway of the threading. This will remove any lint, debris or extra thread you have in there that is not allowing your top tension to close properly. Now rethread the machine making sure you are through your guides, take up lever and topp tension correctly. Before you are going through your needle keep gently pulling your thread while lowering your foot. You should feel the top tension beginning to grab your thread. If not, you are either in front of or behind your tension discs, so rethread it again. If that does not work then there may be thread around the take up area. Remove the face plate (cover on upper left end of the machine) Inside there you will see the connecting mechanism for the thread take-up. There will probably be thread wrapped around that, which will need to be cleaned out and the mechanism and lubricated.

If you clean it out and the issue is still similar then the issue may be that it needs the timing adjusted too.

I haven't heard from you. Was the information I provided helpful?