Over locking thread
If you find that your over locker (serger) is not sewing properly, you may find that its the quality of your cotton. I did an alteration recently and the customer brought the cotton with her. She wanted a rolled hem on a piece of chiffon that she would wear as a wrap. Ordinarily it should be a simple exercise, but this cotton was very coarse and it caused the tension to become very tight. I broke 3 needles before I finished the job. I changed the cotton to a good quality cotton (different colour) and the machine sewed perfectly.
So use quality cotton rather than going for the cheaper ones.
Happy sewing
Judith aka genie