How far away from the flagpole did you put the box? You don’t want any predator being able to reach or jump from the pole to the box. Assuming that is, that the pole the box is on is baffled.
Good luck getting the blues back. How far apart will the two boxes be? I had to go at least 25 feet apart to get to different birds nesting in paired boxes, any closer and the first bird to claim one box would protect both boxes. Some people to have success though having them closer than that.
Donna in WI