Hard Year for the Bluebirds

This year's bluebird increase has not been as much as last year. We had one nestbox that did have two of its babies survive and enter the world outside the box. That box last year had several families, but this year only the one. One of the other boxes had a nest and six eggs. Two of the eggs hatched then within a week some type of predator invaded the box and took all. I am assuming that I need to install a better snake guard.