We are running Exchange 2013 in a period of co-existence with Exchange 2007. This issue just started recently and does not happen consistently. When you add users to scheduling assistant you often get "No Information" very randomly.
But if you uncheck and re-check "Send this Meeting to this Attendee" or remove/add the user several times, then the Free/Busy Information will populate again.
Originally I suspected this is an Outlook issue, but strangely it is happening within OWA as well. So this is most likely a server issue.