Mottled egg yolks (yolks that are not uniformly in color but seems to have a "mottled" appearance, an uneven distrubution of the pigment) can be the cause of an increase in early mortality, as the vitelline membrane of these eggs is not in optimal condition. This drop in hatchability due to an increase in early mortality or appearant infertility can be quite severe.
The reason for this mottling is not completely clear, but disturbance of the breeder flock by stressors as blood sampling, over mating etc is believed to be a factor of influence. Also traces of Nicarbazin in the feed can induce mottling.
Too high temperature in the laying nest or not frequent enough collecting of the eggs during warm periods might also be a factor that can result in an increase in mottling and an increase in early deads, resulting in a drop in hatchability.
Viral infections could play a role as well, but until sofar there has not been an indication for this.