Dead blue tit nestlings second year running

Hi - we’re in North Cornwall and now, for second year running I have found 5 just-feathered nestlings dead in our garden nest box. The same happened last year but to 7, and the brood were nearly fledgling size.
  I have noticed a marked diminution in butterflies and other insects here this year, and spring has been colder, with more rain - is there anything one can do to help?

Very sad.