The cause of the problem in this case is entirely obvious in the photo, because I'm experienced enough to recognise it at this point (in other words - I have made this error more than once), which made me think I should probably write a bit about the things I've learned in troubleshooting sewing machine issues. A bit of 'Zen and the Art of Sewing Machine Maintenance'.
I've worked with a few sewing machines, and owned 5 (not counting overlocker). There are some general problems I've encountered over and over. I was lucky that I had a sewing machine shop with a wise, if somewhat judgemental lady who would tell me straight (maybe a little pointedly) what I was doing wrong.
So here are a few tips for trouble shooting, and avoiding frustrating stupidness whilst sewing.
You should always listen to your sewing machine. It has a particular rhythm. If the sound changes, and/or it sounds like it's struggling or unhappy, STOP. Because there is a problem. Don't keep sewing and think it'll work itself out or it will be fine if you just pull it through. It won't. It'll probably eat your fabric. Stop, clip the threads and ease it out. Get it working on some scrap fabric before you go back to your fabric.
2) Dirty is not good.
In the case of your sewing machine, dirty is never desirable. I wrote a post a while back about cleaning your machine which is here. If you keep on top of it, a quick vacuum of the bobbin area on a regular basis can also make a difference. In case of the photo above, the visible dust on the foot makes it clear that there is lint building up in the machine. Little balls of lint build up between the moving parts in the bobbin, and it can cause skipping and bent or broken needles because the pieces aren't sliding past each other or meeting with the precision they need to work properly.
A lump of this in the tiny workings of the machines would be as disruptive as walking around with a marble in your shoe.
3) Try re-threading your machine. With the presser foot up.
The top thread runs through between two tension disks. These disks hold the thread firmly so that the movement of the take up bar pulls the extra thread out of the bobbin, and not just more off the reel. If the thread misses these disks, it wont work, and you'll have problems like loose loops on the back, which can form big messy knots. Lifting the foot releases the disks so the thread slides through easily. Once you've threaded, drop the foot and tug on the top thread. It should resist when the foot is down and slide when the foot is up.
Also, there is no bobbin tension disk. So even when it appears there is a tension problem on the other side, it is likely to be the top tension.
Do some test stitches using the handwheel before you put your foot down, partly because if there are any issues you won't put undue pressure on the motor, and also because you will be able to feel any resistance if you're turning the hand wheel.
4) Use a new needle.
Or the right needle for the fabric you have. If you are getting little pulls or again skipped stitches, you may be using a blunt or inappropriate needle. Check what is best for the fabric you are using, and get a fresh one. I always do a quick run on a scrap of fabric to check that the needle is suitable, before I start on my cut pieces. It takes seconds and is well worth doing.
5) Try switching to a few different stitches.
Sometimes the machine may not be properly set on a stitch, and changing from stitch to stitch can help align it. Or starting with a basic straight stitch and working through more complex stitches.
Sometimes I find running through a few scraps of different fabric seem to help. For the life of me I don't know why.
7) Cheap thread is not worth it.
Sometimes cheaper threads cause all sorts of hassles. It isn't as smooth or as strong. Get good thread. It will be worth it.
Hopefully that's sufficient to sort most problems. Generally it's what I look for if I have a hiccup. Granted the machine may actually be damaged. But it pays to have somewhere to start, and maybe avoid trudging down to the shop and having to wait two weeks for it to be repaired if it's just lint, cheap thread or user error.
What about you, dear readers? Do you have a machine fault you keep coming across. What are your top tips for dealing with a moody machine?