Under "settings" there is a section called "privacy". Under "privacy there is a section to "allow users to....." and one box is receive private messages. The preference can be set to "members only", "people you follow" or something else I can't remember since a minute ago when I looked. Make sure that setting is "members only", which means members and staff. Any other selection will pare down the list considerably and not allow private conversations to be started between the whole membership.