Machine will not boot to automation

We current image 30 servers on a weekly basis, and for obvious reasons these are done out of hours. Ever since we upgraded to DS 6.9, three of these servers suddenly will not boot to automation without manual intervention.

I have ensured that the AClient was updated (it was), and that the machine is set to PXE boot from the correct NIC (it is).

If I run the imaging job AClient reboots the machine, the PXE boot process goes through correctly and when it gets to the boot options menu, instead of selecting the default (boot to DOS automation) it just carries on with a normal boot. However, if I press F8 at this point and then enter to select the "boot to DOS automation" option (it is already highlighted as it is the default option), the image job runs as it should.

The actual image job is the same for all 30 servers and uses the same bootworks settings, and nothing has changed on the machine so why would these three servers just refuse to image?

Hope someone can shed some light on this as it is getting really frustrating.

Thanks