Thanks for the reply, Konstantin.
We've had both true plenty of times. We've relogged, moved around a bit, have pvp off and are definitely in the same region. And while we still get "not suitable" to each other, random folks are inviting us to coop quests (which we decline of course as we want to coop with each other).
So each of us can be invited by other folks. We just can't seem to invite each other.