The PXE Boot option fails to load when you set NBS General Settings > Redeployment (Managed Computer) Menu to a specific PXE boot image. This seems to work for a while until you rebuild the selected PXE boot image, and then the menu will not come up.
DS 8.x
Set the Redeployment (Managed Computer) Menu PXE Boot image to "Next Device (BIOS/EFI)" and save the changes. This will reset whatever is not allowing the menu to come up.