Sounds like you know what you are doing so not sure I can be of much help but if not already done?
Is the Red LED on the hash Board lit?
Try the hash boards in the problem miners singly?
Try turning down the frequency. Drop it right down to say 150MHz just to see if it hashes at all?
Check the Kernel Log around 33 seconds after reboot an make sure the chains have really been found and that it has not just defaulted to 30 chips.
!!!!soft set defult asic num 30!!!
as opposed to
chain0 total asic:30
After that you are into making measurements on the board, but the hash board is almost certainly faulty.
Rich