I am experiencing this as well (top front center for me). I had a Komaza hair analysis done and she told me that my "off" section will always be an indicator of if something is not 100% right internally. I was advised that when the internal issue is resolved, that section will begin growing like the rest of my hair.
Unfortunately my doctor refuses to give me a copy of my blood test unless I pay another copay. I have to go in there and do what I can to get a copy of my lab results so I can see exactly what they test or don't test for. I'll definitely be switching doctors for this year's physical.