Blue tits disappeared?

Hi all, I set up an rspb nest box in February and got interest straight away from a pair of Blue Tits. They progressed as I expected and started building their nest eventually. They spent 2 or 3 weeks building and I caught them mating a few times. But then we noticed they weren't there as much and were only seeing them now and again. Now they seem to have completely deserted the nest although I don't think any eggs were actually laid but can't be sure as I don't have a camera inside. Now we've got a couple of Great Tits sussing out the box. What has happened to the Blue Tits?