Although martins have been known to nest in houses and gourds painted other colors, white housing seems to attract them best. White reflects the heat of the sun, resulting in fewer nestlings lost to heat stress. White also is believed to best enhance the male martins courtship display. Housing opened up too early. Contrary to popular belief, the scouts are not looking for new breeding sites, they are simply the first to pass through an area on their way to their previous nesting sites.
Older martins come first, then younger ones.
You allow other species to claim the housing first. If your site is new and not already established by martins, and you allow other birds to occupy their housing, it rarely will attract martins. Birds are extremely territorial, and will defend their sites against other birds. Housing placed too close to tall trees or in yards that are too enclosed. This is the single biggest reason people fail to attract martins.