Thanks for the great photo!
There are many reasons why that house would never fill, including the fact that pairs dominate multiple cavities that are closely spaced.
The state of Louisiana has a much higher population density of martins than Ohio. This 620 compartment house in Louisiana has attracted at most 300 birds, or about 150 pairs:
http://www.purplemartin.org/update/Finney.html
The above article has a great interview of the elderly, lifelong martin landlord, including this exchange...
Q:
What have all these years of sharing your front yard with martins taught you about their habits? Are they good at catching insects?
A:
They're the bug-catchingest things I've ever seen! The only kind of insect that I've actually found them with though, and they use a lot of them, that's these little mosquito-hawks [dragonflies]. A lot of times the martins will bring them mosquito-hawks in to the little ones in the box. Of course, they all got their heads stuck out there and they all grab it. Then the old bird will take off and the young they'll drop it. I've seen a time when there'd be, oh, maybe 100 mosquito-hawks around that box, ya know, where they fell out onto the ground. The parents won't pick them up after the young have dropped them, either.