There are several factors that may lead to failure of fertilisation of eggs in chicken.
One of these may be the ratio of cocks to hens in the flock.
Among chicken, sexual activity is initiated by males and this is done through courting.
Chicken are polygamous, but certain cocks and hens mate regularly.
For effective mating, ensure that the ratio of cocks to hens is 1:8-12. If the number of hens is higher than this, then the cocks will not adequately fertilise all of them.
Under the circumstances, you will experience the scenario of few eggs hatching. A cock may serve 10 to 30 hens a day depending on their availability and competition from other cocks.
As mating continues, the volume of semen and number of sperm cells decreases.
If males are removed from the flock, some fertile eggs will be produced for as long as four weeks, but the percentage of fertile eggs decreases each day. It is also advisable to have a second young cock in the flock to help out the old one.
Old cocks become heavy and lose interest in mating. Young athletic females also tend to weaken them, which diminishes the frequency of mating, hence failure to hatch.
