I used to think that only sheitels got knotty at the nape of the neck & it was quite annoying
when I stopped wearing sheitels and let my curly hair go wild - I had the very same experience - at the nape of my neck the hair would get knotty
what's the difference then between sheitel & hair ... before my sheitel times I was always brushing my hair [because I blow-dried it straight - thereby not ever letting it get knotty
perhaps if people brushed at least the underneath - near nape of neck - hair; your sheitels would not be getting so knotty either