Believe it or not sisterlocks breaking or snapping off is very normal, so do not be alarmed if you lose one or two locs at some point. Things such as coloring your hair or twisting your hair into cornrows can cause even further breakage. In order to prevent this, you have to be extremely careful with your hair especially if you style it or color it regularly.
Your locs breaking or snapping off in the middle or end of your hair is normal, but thinning or breakage at the scalp is NOT. If your hair starts thinning at the scalp then it will cause an immense amount of breakage. Some reasons this could be happening is because of age or postpartum. If this happens it is recommended that you go see help about this problem.