I've had trouble with this in the past and it has normally been subnet related, although we do have a few sites with complex subnets working ok.

The only other possibility is that it doesn't like the nbi that has been uploaded. Are there any spaces in the nbi name?

I have tried the nbi file that I've been using successful from an OSX server plus I created a new one just in case.

The file name was casper1.nbi so less than 8 characters. no spaces or non alphanumeric characters.

I will simplify the name further and see if that works.

Thanks for letting me know that the subnet should work.

Created a new nbi file called C1.nbi using Casper NetInstall Image Creator 5.0 from a base OSX 10.9.5 dmg created using the "Build OS Package" in Composer 9.62.

Settings used were;

Image Name = C1
Image Index = 1101
Enable this NetInstall Image = True
Set this NetInstall Image as the Default Image = true
Compress image = true ( tried false last time )
Added the 9.62 casper imaging app
Created Casper Preference file
added the correct JSS url

NetBoot will not enable. In /var/log/auth.log on the NetSUS appliance I can see "setnbiimages C1.nbi"

Also tried a simple subnet but NetBoot will not enable

I'm not doing much with NetSUS at work as our (currently) supported client range is too broad (benefits from multiple NBI's and filtering). I did mess around with NetSUS 3.02 briefly at home this weekend. vSphere 5.5 with Ubuntu 14.04 and CentOS 7. Ubuntu worked straight away with NetBoot enabling/disabling as expected. CentOS was quirky and would enable but not disable NetBoot. NetBoot on CentOS 7 also didn't remain enabled/persist across reboots in limited testing (dhcpd.service likely not enabled @ boot)

I am admitting defeat after spending a lot of time trying to get this to work.

I have gone back to a physical mac running server which works reliably all the time.

The NetSUS concept is a great idea but after many hours of playing with it I could get it to work occasionally but I never worked out why and it wasn't stable.

Restarting AFP seemed to work 1 out of 10 attempts. eg restart AFP then enable netboot.

Simply to unreliable and time consuming for a production environment with, I'm guessing here, complex subnets.