What brand and model of machine? Is it a vintage machine? If vintage, it may need oiling.
If you use dental floss to clean between the top tension discs, just make sure it's *unwaxed* dental floss!
To me, it looks as if the bobbin thread stops getting captured correctly. I would double-check the bobbin (make sure there isn't lint or thread stuck in the spring area) and make sure the bobbin is threaded correctly. Is it a drop-in bobbin?