female boas cannot be bred every year. the length of gestation, and the number and size of the babies takes a pretty big toll on them. they will often go several years in a row (esp if they are kept well fed, and they rapidly regain the weight), but then their bodies need a break...and take it. Males will definitely still do their part; but either the females just don't ovulate, or it doesn't take. In all likelihood, this is what happened with your friends' snakes (you don't mention the size of the breeding group, or the breeding history, so I am guessing that this happened to a relatively limited #). there could certainly have been other issues: ie changes in the cycling process