Remember that the bobbin works on E.V.E.R.Y. stitch. No so the upper thread course. Your problem is gonna be in the bobbin area 9 out of 10 times....or even MORE on a well adjusted machine, because most of the upper thread paths are doing NOTHING, all day long, every day. Fuzz is going to build up under the springs of the bobbin cases and with additional wear and so forth on the bobbin, its always the first place to look when the "tug of war" that happens on every stitch is gone all whackadoodle.
Ask yourself, "who's winning this tug of war? The upper thread or the lower thread?" In your pic, the upper thread is overpowering the lower thread. But your upper thread has "fresh legs" (probably) since for the last week, it hasn't been working every hour of every day. See?
That's just how I try to 'splain it to the help. Doesn't work worth a hoot, since I'm surrounded by wimmens. They glaze over pretty quick on mechanical stuff until they finally "get it". Not a one of them can master lefty loosey, righty tighty...left handed or upside down. Sheesh! We had a gal who could, but she went POOF a year or so ago....
Your problem presents to me as low bobbin tension. Sew a different color and verify, but don't be yanking around on those upper tension settings. Oh, maybe 5 to 10 times out of a 100. If you are a noob, the Towa Bobbin Tension Guage is a good tool. After some experience, it'll sit on the shelf, mostly. UNTIL you need to train somebody! Then you'll get it out again. Oh yeah. Trust me.
And we LOVE the Magna Bobbins too! Light years ahead of paper spools, that drag and pinch and mess with your mind.
Its just a machine and it operates under the laws of physics. Few mysterious anomalies actually exist down the stretch..You'll figure this out!
