There are several possible explanations for this:
Some machines are slow to start; wait for them to start. The installer stops waiting for hosts after some time, so the machine may still return.
If the hosts do not return, they may be stuck in the reboot. Windows machines have been stuck during boot due to a broken MBR, or a missing keyboard. Usually, looking at the machines console lets you know what is wrong.
If the upgrade fails, run the installer again. It helps to reboot the problematic machines before running the installer again.