Thread break detection is done by the check spring. If the check spring is sitting against its post, then the machine assumes the thread is broken. If the machine consistently is not recognizing thread breaks on a particular needle, look at the check spring area. Be sure the spring can move freely. Clean the spring and the post to be sure it can make good electrical contact. You can also try loosening the check spring slightly. If the problem persists, or is occurring on more than one needle, you may have a different problem and need to contact technical service.