Try re-threading the machine; make sure the thread goes through all guides.
There may be burrs in the needle's eye, on the thread guides, needle plate or the hook. Replace the needle and try buffing the thread guides and needle plate. Buffing may alter the timing, so it's a good idea to replace a damaged hook.
Because it dries out faster, the hook needs frequent lubrication. The hook assembly should be lubricated every 4-8 hours of machine running time.
Needle to small
You may have to change to a larger needle
Too much Adhesive spray
You should be using only Embroidery designed adhesive sprays, never over use them. Too much adhesive build up on the needle can cause them to drag and break threads. If this is happening you should examine the way you use your adhesive spray
If your machine isn't timed properly, it can cause many probems in addition to thread breaks. Such as broken needles, poor stitching or sometimes no stitching.
Thread can become brittle with age and long exposure to light, air and heat. You should store your thread in a cool, dark place to lengthen shelf life.
A bruised cone of thread can occur if the thread falls on the floor, causing the lower winding of thread to loosen and catch as the thread spools off. You can take off the affected part of the thread or replace the cone.
Some of the designs increase the stress on thread. You should lower the machine speed as needed
Lint build up
A lint build up on the tension discs, needle plate or bobbin cases can cause breakage. These areas should be cleaned frequently with air or a brush.
This can happen after the needle hits a very hard object such as a Hoop, needle plate or the hook. If this happens you have to replace the needle and check for burrs and the timing.
If the tension is too tight, it can cause pulling, puckering and thread stress. If the thread is too loose it can cause the thread to pile up and loop. You have to adjust the tensions to achieve a flat smooth look.
Using Tape for loose ends
Using tape can leave a residue and this can cause friction and thread breakage.
Holding the thread
If you hold the loose end of the thread when you start up the machine and you are pulling the thread too tight, it can cause the needle to bend or break and can also cause the thread to break.