So many things could have gone wrong.
- Low humidity or temperature for a prolonged period.
- Low humidity during hatching.
- High temperature during hatching.
- Nutritional deficiencies.
- Breeder diseases.
- Poor ventilation.
- Inadequate turning during first 12 days.
- Injury during transfer.
- Prolonged egg storage.
If the chick was weak and died
- High hatcher temperature.
- Poor hatcher ventilation.
- Excessive fumigation.
- Contamination.
I often suspect nutritional deficiencies in the breeder flock if I know the incubation went well.
Death to fully developed chicks could be a deficiency of vitamins D, K, thiamin, riboflavin, biotin, folic acid, B12, manganese, zinc, magnesium, molybdenum or imbalanced amino acids.
Many feeds are sufficient to produce eggs and maintain bodies but not necessarily to produce viable hatching eggs and healthy embryos.
The roosters' nutrition is important for the viability of his sperm. Roosters that have been fed layer feed have lower sperm vitality.
The hens have to deposit enough nutrition in the egg to feed the growing embryo.
Most people ignore these things as possible problems in hatching.
Commercial feeds, though good, aren't always perfect. The vitamin and mineral supplements may occasionally be omitted in a batch.