When using booking assistent and roomlist meeting rooms does not always show in the list as expected. One reason for this is that user make a default invitation and by default the meeting time is this morning witch is already passed. Another thing is that default working hour for the meeting room is 08:00 to 17:00 – outside this the room will not show in the roomlist.

Use get-mailboxcalendarconfiguration to get workinghours for a room / mailbox.