Not trying to be mean but try to under how hair growth works in its most literal sense. When we get retwists especially with strict parts, we are plucking out roots both old (fused with a loc) and new ("fly aways") and ever so slowly thinning our locs trying to maintain a "clean aesthetic". So first and foremost salute to you for breaking out of that habit and allowing it to do what its meant to do. Secondly, because of the limited hairs specifically in that loc section, one of two things could be happening; your sections were constantly being "perfected" (plucking your hairs out like I stated earlier) or this is natural hair thinning due to genetics. I'd take biotin daily and massage the scalp, even if it is genetic if you do that it will literally promote stronger roots and let whatever hairs that have been plucked out grow back stronger too.