The bobbin area needs a full cleaning. It's a fairly easy job, but can be a bit tedious and there are a couple of small screws and springs involved with removing the bobbin assembly. It looks pretty gunked up and it needs servicing. Are you up to that? Or maybe a quick visit to your local service person? I would imagine the rest of the machine needs servicing too. And possibly a new friction pulley, if the machine hasn't been used in a long time...
Hope this helps.