Bluebirds vs The Box

For some years we have offered a nest box on a post, one designed for bluebirds.  Only problem is that the bluebirds have rarely used.  And this season not at all.  And even worse---apart from an abortive Tufted Titmouse nest effort the box has been ignored---even by House Wrens!  I'm baffled.  The thing is as far out in the open as I can go in limited acreage and is a respectable distance from trees.  All I can imagine is that I have a pointing in the wrong direction.  It faces north.  But enough takers in the current mode have used it that I am doubtful.  I think remedial attention must be made this fall.  What to do?  There's much to think about.