Blue tits dead in box

I am a new member.

For the last two years I have had blue tits nesting in my nest on  garage wall. Last year despite the building of nest and plenty of activity. When I came home from a 1 week holiday I found activity had ceased and after a couple of days I checked box to find no signs of eggs or birds in box although sure there must have been at least two eggs from what I could see.

However this year even worse, Again more bird activity and checked box for nest all fine but did not disturb nest to check on any eggs. Birds in and out carrying worms and fat ball food. Just come back away 1 week no activity and found 6 dead chicks in nest no feathers no egg shells cold and maggots looks like parents diserted box. 

Can you explain what may have happened 

Alan