Lois left a comment about having intermittent breaking of her top thread while free motion quilting on a recent post. I answered her in the comments, but wrote a good sized equivalent of a blog post. I figured I might as well re-post it for you.
So, here's my checklist for thread breaking with a few additions:
1) Quality thread? Not too old? Not rayon (Rayon is a beautiful thread, but it takes gentle treatment and does break frequently).
2)Check your needle for snags, sized appropriately, etc. Some threads do better with a top stitch or metallic needle. Sometimes you can get a bum needle. But sometimes the bigger eye of a topstitch needle can cause sloppy stitch formation.
3) Check your stitchplate where the thread goes into the machine to form a stitch. Any snags or rough spots? Since you're going in all directions, the thread can contact a rough spot that hadn't been an issue with regular stitching.
4)Is it directional? Only happen as you stitch backwards? Happens to me sometimes. The needle can actually pierce the working thread (thread takes many passes through the machine before it is finally made into a single stitch, piercing it will break that working thread pretty quickly. Try to limit how much you stitch directly backwards, especially as that's where the needle is most prone to flexing.
5) Thicker, rougher threads, even some quality cottons can be 'grabby' and shred if there's a lot of stitching on top of stitching (backtracking or really dense designs).
6) Sometimes it can be a timing issue- if the top thread seems to be getting split and parts are worked into a few different stitches while a ply isn't getting stitched. The hook is splitting the thread as it's being formed. Tends to happen on thicker threads or lower ply thread (2 ply vs. 3 ply).
7) Check the upper thread path. Sometimes a slippery thread can flip itself around something as you stitch, work itself out of the take up lever, etc.
8) This point wasn't in my original response. Are you also having tension issues? A too tight top tension can sometimes cause the thread to break.
9) Another tip that wasn't included earlier: Check your bobbin case for nicks, scratches, rough spots too. The top thread has to pass around the case in order to form a stitch, so it's got to be smooth and undamaged.
10) My last tip, and another one not included in my comment, is to check how the thread is unwinding from the spool. If it is a straight-wound spool, it may not like the extra twist it is being given by spooling the thread off the top. If it is a slippery thread, it may be getting tangled around the spool pin and/or benefit from a thread net.
I also want to include a few more tips gleaned from working at my Janome dealer's shop part-time.
Make sure you are using the proper bobbin for your machine! You would be amazed at how many machines we see whose main issue is the wrong bobbin! It's easy to grab the wrong bobbin, especially at a big-box store's sewing section or if you have had multiple machines. The machines may move on, but the accessories and bobbins tend to stay.
I also recommend getting genuine bobbins from your machine's manufacturer. Ask your dealer if the bobbins you are being sold for your machine came from the manufacturer or if they are generics that fit your machine. There are generic bobbins out there with the same dimensions (or "close-enough"), but I always have less trouble with my Janome bobbins in my Janome machine. I also like buying the colored bobbins from Janome so I can separate the thread typed in my bobbins. I recently got blue bobbins
and they also have pink ones. Take it from me, I buy generic everything most of the time, but not bobbins.
Don't forget to raise your presser foot when threading your machine, so that the thread slides into the tension disks properly. I also raise my presser foot first when I am changing my top tension.
If you have a drop-in bobbin machine, have you ever noticed that even when the case is correctly seated, there's a little bit of wiggle room? It's supposed to be there! That bit of play allows the top thread to pass around it as the stitch is formed. As you stitch, if you hear the bobbin rattle, you might have something impeding the threads path around the bobbin case. Check it for lint or debris build up. I hear this sometimes when my bobbin is running low, the tail end of the bobbin thread has come unwound a bit and is interfering with the stitch formation. If a long enough of it comes unwound, it will join the top thread in making the stitch and *whammo* the bobbin with be tied in place.
Lastly, my experience has taught me that there will always be a few thread breaks occasionally. Even a long armer will have breaks occasionally. But if it's happening after stitching no more than 5 feet or so, there's probably something you can do to fix it.