I've been scratching my head over this also. If it persists might be something for Kevin to look at. It's over my head why it would be working fine for everyone except a couple of people unless they have something in their set-up set wrong.
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.