An incorrectly threaded machine can cause thread to break.
Decrease the tension on your machine
The tension may be too tight for your thread.
Reposition the spool of thread on the spindle
You know that little nick in the edge of a spool that’s used to hold the loose end of the thread? Your thread may be catching on this nick as you sew. Place your spool so that this nick rests against the bottom of the spindle.
Reinsert your needle
Make sure it goes all the way into the holder and that it’s positioned correctly, usually with the flat side away from the bobbin, but consult your machine’s manual to be sure.
Change your needle
Your needle may be bent or dull. The eye of your needle may have a nick in it that’s cutting the thread as you sew. Or the eye of your needle may be too small for your thread, in which case, change to a larger needle. And make sure your needle is appropriate for the weight and type of your fabric.
Change your thread
Your thread may be weak. Or your thread may be fuzzy and irregular, causing it to catch in the eye of your needle as you sew. Cheap thread tends to be weak and fuzzy, so make sure to use a quality, brand name thread. And if you’ve been sewing with an older cotton thread, which can dry out and become brittle with age, try a newer thread.
Check your presser foot for damage to the needle hole
A burr in your presser foot’s needle hole may be cutting your thread. Gently smooth any rough spot you find with an emery cloth.
There may be a burr in one of your thread guides, in which case, you may need to have your machine professionally serviced to have the part replaced.
If your lower thread is breaking:
Reinsert your bobbin
Your bobbin may not be seated properly in its case.
Rethread your bobbin
Your bobbin may be threaded incorrectly or the thread may be tangled.
Remove some of the thread from your bobbin
If your bobbin is very full (if it’s hard to insert into the case, it’s too full), this may be keeping your bobbin from turning smoothly.
Rewind your bobbin thread
If your bobbin thread is wound unevenly, this may be keeping your bobbin from turning smoothly.
Try a new bobbin
There could be a nick along the edge of your bobbin spool that’s catching your thread as you sew.
Clean the lint from your bobbin case
Lint in your bobbin case could be keeping your bobbin from working properly.
Check the needle hole in your throat plate for rough spots
A burr in the needle hole of your throat plate (usually caused by the needle hitting the throat plate) could be cutting your thread. Gently smooth any rough spot you find with an emery cloth.