Thank you for the details.
Just a note here, please make sure that you are booting the same type of machine from which you have created NetBoot Image.
NetBoot images are very much coupled with hardware type unless you have applied combo update before creating NetBoot image.
So if you are creating NetBoot image of iMac 12,1 then make sure it used to boot same type of hardware i.e. iMac 12,1
Thank you,
Anil