If you have eliminated all of the design related causes for thread breaks (small stitches, excessive overlapping, etc.) and, of course, the condition of the thread itself and/or the way it is wound on the cone, then it is time to take a look at your machine to make sure that its condition is not causing them.
Burrs on the needle
It is a good practice to replace all of your needles on a regular basis, based upon how often you use your machine. A maintenance check list attached to your machine can help to remind you what to do and when. if however the needle damage is as a result of the needle bending or breaking and hitting the presser foot or needle plate replace it even if it appears to be fine.
Presser foot
Check for a bent presser foot epecially if you have hit the hoop at any time.
Needle Placed backward
Some times we get distracted and the simplest of chores suffer the most. Check the needles and make sure that the needle groove is in the front of all of them.
Incorrect needle
Embroidery machine use DBXK5 needles, while the points and sizes vary the needle type is always the same.
Bobbin Thread
As the bobbin thread nears the end of the spool it sometimes varies in tension, if this is the problem replace it even if it is not completely empty.
A groove or burr around the hole in the needle plate
If you feel around the hole in the needle plate and find a nick or abrasion either replace the needle plate or even out the area with an emery cloth.
Lubrication
You must always make sure that the hook assembly area of you machine is well lubricated and free from lint. both of these factors will cause undue friction and the area to heat up and damage the thread.
Needle Depth
follow your machine maual to check the needle depth to make sure that the thread is looping properly and not causing a false thread break.
Incorrect Thread Path
Check the thread path to make sure the machine is threaded correctly missing one step can cause uneven tension and thread breaks.
Thread or bobbin Tension
Take a look at the backside of you embroidery and check to make sure that you see 1/3 bobbin thread on your column stitches, if you see more loosen top thread, if you see less tighten top thread.
These are some more causes of thread breaks during the embroidery process. As you become more experienced you will find that there is a more prevelent cause of thread breaks for your particular application and will learn to start with that cause first when determining the cause of your problems.