Weakened roots can be caused by a few different things. Dreadlocks are augmented - thickened - by capturing and knotting the hair that is naturally shed from your scalp into its nearest dreadlock section.

If you have started with very small sections, it's possible that the weight of the lock is now too much for the small amount of hairs that anchor it, breaking them slowly. In this case, you may consider joining your weakened lock to its neighbor, giving it a larger base. If your scalp is thinning or balding, you are no longer replacing shed hairs with new ones, and the dreads will eventually be unable to support themselves.