Solution
There are several possible explanations for this:
Reason 1
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.
Reason 2
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.
Reason 3
If the upgrade fails, run the installer again. It helps to reboot the problematic machines before running the installer again.