Egg laying is very physically draining on the hen. If the hen is not in peak health, then she can have problems with laying and often won't want to brood and raise a clutch. Raising a clutch of chicks is also very physically draining on the hen. Ensure that the hen is in good health prior to the breeding season by seeing to proper nutrition, worming and control of any underlying health conditions. The other common causes of failure to brood relates to the nestbox or log.
Red mite is a common nest contaminant.
These parasites are blood suckers and will prey on the hen when she is in the nest. Many hens will refuse to brood and leave their eggs if there is nest contamination. Carbaryl powder can be used prior to the breeding season and between clutches. Dust under the nest litter and around the edges of the nest. In water medication with Ivomec can also treat infestations.