I have Three Birdhouses And No Birds Living In Them, What Am I Doing Wrong?

About seven years ago, I bought three birdhouses. I decided to paint them (which was probably a mistake), and waited a couple weeks, then put each birdhouse in a different tree in my backyard. I really didn't expect any birds the first year, but thought after a year or two, they'd notice them and I'd have three new families living in my backyard. The houses themselves are fairly small, and the hole is fairly small too, so maybe that could be the problem? I do have one house that has a nest in it, but no birds, just an empty nest. I have lots  of birds flying in my yard, and sitting on the tree branches, but they don't seem to be interested in the birdhouse. Does anybody know what I did wrong, or something I should be doing that I'm not?

The birdhouse pictured is a lot like mine, except the hole is a little smaller, and it's painted all red. To help me, just leave a comment in the comment box. Thanks.