BLUEBIRD VACANCIES

I'm very confused. I thought I'd come back from the Big Apple to find lots of nests and possibly bunches of sky blue eggs. Not so.. Out of the nine houses along our trail, I have 2 nests, one with 4 eggs and the other has 2 as of this morning. The remaining 7 houses are completely empty! Nothing remotely resembling the start of a nest in any of them. I don't know if this latest cold snap has something to do with it or not. Something is eating the mealworms I place on top of each box, and I am assuming it is bluebirds. I guess it's just too early for the rest of them.