Blue Tits

Blue Tit  or possibly great tit made a nest in camera box end of March but has not returned 

This is the 1st time this has happened after 9 years of successful hatchings

we normally have swarms of blue tits and great tits in the trees but hardly any this year but we now have swarms of sparrows

so sad not to watch the nest progress to a happy ending of fledglings  

what could be the problem ?