Workstations will not start imaging session

We have a recurring problem at multiple locations in our school district.
Here is how it goes: First, we create a multicast imaging session in a
server object and specify workstation names to receive the image. Then, we
boot the workstations with PXE enabled. Sometimes, the workstations will
completely ignore the imaging session.

Such is the case today at one site. We have tried using criteria rather than
workstation names, to no avail. We have deleted the workstation objects and
let them get recreated, then added them to the imaging session again,
without success. We have run DS repairs, unloaded and reloaded ZDM, etc.

We are currently running ZDM 701 on NW65SP5.

Any ideas?

Rick P