Is it normal for some of the eggs in the nest to not hatch or for some chicks to die?Updated 10 months ago

Sadly, yes. It's quite normal for a few eggs to fail to hatch, or for some young to die. Blue and Great Tits lay up to 14 eggs to allow for such losses.

Cold weather and/or food shortages may lead to nest desertion, or for only the strongest of the chicks to survive. The death of one parent or interference from animals or humans may also cause nest desertion.
Remember. if there are unhatched eggs in a nest box, these can only legally be removed between August and January.