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 awhile until you rebuild that selected pxe boot image. Then the menu will not come up.
Set the Redeployment (Managed Computer) Menu Pxe Boot image to "Next Device (BIOS/EFI)" and save changes. This will reset what ever is not allowing the menu to come up.