using zen imaging ver

In the last few weeks, our images have ceased working.

What I mean is, that when we manually (we only do manual imaging, no
automatic) image a machine by going into the pxe imaging menu, after the
image successfully applys to the target machine, when that machine
boots, it gets a little bit into the boot cycle (pre-os) and then:

a)if it is an HP laptop (several models/ages) it reboots again. Over and

b)if it is an HP desktop (again, several models, several ages) it hangs
at a black screen with a blinking cursor.

this is true whether the image was created months ago (and used
successfully for all this time) or if it is a newly created image.

How do I begin to even trouble-shoot this? What logs are there to look
at, what diags might I run?