I have a Elna excellence 7600. Why does thread keep breaking while chain stitching fabric?

Thread breaks
first check to see if there is an actual thread break, then check the threading-both needle and thread
Sometimes the machine will signal a thread break when there really isn't one, at least not technically. For example, a mis-threading can cause the thread break sensor to think there is a problem because it is not sensing the proper thread tension. This can also happen if a loop of thread forms suddenly resulting in slack in the thread.
a thread break signal are really a "thread out" indication most often triggered by an empty or low bobbin or the needle becoming unthreaded.
rethread the machine and correct any low thread conditions.
Actual Thread Breaks
Examine the thread break. Is it a clean snap or is it shredded?
Next, examine the thread path. Did the thread get caught on something
A common problem with embroidery thread is that due to its slippery nature, it can slide down the spool and get hung up underneath causing a sudden jerk on the thread. Rayons will probably just snap at this point but some poly threads may be strong enough to resist breaking and instead flex the needle resulting in a broken needle.
Metallic threads often cause more breaks and shredding because they tend to form loops and kinks as they come off the spool. This is exacerbated by small spools with narrow diameters and short thread paths from the spool to the first thread guide. A simple solution for this is to extend the thread path.
Thread Factors
Lets look a little closer at thread. Embroidery threads are more fragile than those used for garment construction or quilting. Start with quality thread and know how to care for it.
Embroidery threads can dry out when exposed to light, heat, drafts, and air conditioning. They can become "bruised" if dropped or handled roughly. As they age, they become more fragile. Due to dying and other processes, certain colors break more easily than others. Check this for yourself. Try breaking a white thread and then a black. Also notice that the black is slightly thicker than the white.
Rayon tends to break easier than poly. Metallics are more temperamental and finicky
Needle-Related Possibilities
On a single needle machine, if you are getting continuous thread breaks with multiple colors and you are sure you can eliminate the thread, then look at the needle.
Is it the right size? For 40 wt rayon or polyester thread, a 70/10 or 75/11 works well on most fabrics.
Embroidery needles have a slightly larger eye to reduce friction on the thread. High quality needles have a well-polished eye that won't snag, abrade, or shred the thread. The thread can pass through the eye of the needle 50 to 60 times before it is laid down on the fabric. Any rough spot on the needle or thread path can literally saw the thread in half
The broken needle is the least of your problem! If your machine doesn't stop the instant the needle breaks, it may severely damage your fabric. Needle breaks can cause scratches and rough spots on the throat plate and in the hook area that can shred thread. Until these problems are smoothed out, thread breaks will be your reality.
It is possible for the fabric and/or stabilizer to the source of thread problems. For example, heavy, stiff fabrics can easily strip some metallic threads
If the lower thread keeps breaking, open out the bobbin hatch and check that there is no lint buildup that is hindering the smooth flow of the thread. If there is any lint, remove the bobbin and clean the area thoroughly with a brush. Also check that the bobbin is wound correctly and that the tension of bobbin thread is even. Poor maintenance and incorrectly wound bobbins are the culprit more often..