As I have a large bluebird trail I have been contacted by a number of birders that are not being successful in attracting bluebirds to their boxes.
Most of their box designs were taken off the internet. The hole sizes must be 1 9/16 inches no more no less for mountain bluebirds. Most internet plans are for eastern bluebirds that take a smaller entrance hole.
I hope this helps.
D. Murray Mackay