I'm not sure what triggered it but at some point i became unable to send followers back to sanctuary. every other settlement is available but sanctuary is grayed out. does anyone know what might cause this and how i can fix it.

Is the settlement "full"?

This happened to me with Preston. I couldn't send him back because I've got 16 settlers there already. I believe the max is 10 + Charisma for settlers/residents.

that might be it then i have 15 settlers now in sanctuary i believe. ill try sending some people elsewhere see if that helps