It could very well be a compatibility thing.

Perhaps the motherboards are newer and have UEFI turned on? Or Secure Boot?

Make sure DHCP is running on your FOG server still. Make sure TFTP is running.

And, grab that Dell that worked before and verify your FOG box still works.

Check out this article:

[url]http://fogproject.org/wiki/index.php/Troubleshoot_TFTP[/url]

Please give an attempt to troubleshoot this on your own first. If you still have problems, let us know and we’ll help out.