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The fact that not all the eggs laid in the nest boxes turn into adult birds is part of nature!
The mortality rate in the first stages of life amongst tits is quite high as you will soon discover.
For example, the annual survival rate for blue tits is just 38%, which means that of 100 eggs laid, only 38 produce a chick which will reach the age of one. Only 13% of great tits reach the age of one and then 49% of adults die each year.
But why are the mortality rates so high?
Of the eggs laid, some do not hatch. It might be that they had not been fertilized or the chicks inside have genetic or developmental problems. The presence of the mother during hatching is also vitally important. If the eggs are poorly incubated and are exposed to the cold, it is highly unlikely that they will hatch. The eggs can also fall prey to various predators, such as squirrels.
After successfully negotiating the hatching stage, the fledglings are still not clear of danger. They can fall prey to predators but also become ill, suffer from parasitism or be subjected to climatic conditions that are too hot or cold, etc. Some (or all) of the brood may not survive to start flying if the adults are unable to provide enough food for all their offspring. If the adults cannot bring enough caterpillars to the nest, the principle of survival of the fittest applies. If one parent dies, the brood is often doomed as one adult is not enough to feed all these hungry beaks.
After take-off, the young tits do not have an easy life either and the mortality rate is very high. They have to learn to find food while avoiding dangers, such as predators and windows, etc. Then winter soon arrives which is another story.