Daylight and the seasons
In nature chickens only laid eggs in Spring and into the middle of Summer. The reason for this is because chickens lay eggs to reproduce and during the Spring and Summer months the weather is good and there is more food available, so conditions are perfect for raising chicks. When the days get shorter the hens' bodies sense the change of season and egg production slows down and stops. Humans have been modifying this behaviour and have been breeding the most prolific layers to ensure year round egg production, but given the chance most hens will still do what nature tell them to. Most hens need a minimum 14-16 hours of light per day to fool their bodies into thinking it's Spring and keep them in production. This could be either natural or electric light or a combination of both. Adding electric light to the coop will help keep your hens in production, but keep in mind that this could shorten the hen's egg laying lifespan. Hens are born with a limited amount of egg cells and once those are spent she'll lay no more eggs. If you do decide to add a light have it on in the mornings, so you don't disrupt the hens' natural roosting behaviour. You do not need a bright light, just enough to read a newspaper by.
Stressed hens either lay very strange eggs or no eggs at all. A fright, running out of food and water, disruptions of the pecking order (for example adding a rooster to the flock) and getting moved to a new coop/run all causes stress. When you buy hens and bring them home to a new coop it could take up to 6 weeks before they start laying again. Adding unfiltered Apple Cider Vinegar (ACV) will help your chickens deal better with stress. The ratio is 4-5 tablespoons to a gallon water.
The average laying hen's skeleton contains 20g calcium and one egg represents 10% of that. Hens do have calcium reserves stored up in their bodies, but if they do not get enough calcium from their food for their egg shells the stores will get depleted very quickly and they will stop laying. It's important that laying hens get fed either a proper, balanced layer feed (mash or pellets) or a good quality all flock feed with a calcium supplement like oyster shell offered free choice. It is best to offer layer food "free choice" i.e. have the feeders full at all times, to ensure hens get enough food. Keep in mind that the average hen needs 5 ounces of food and 10 ounces of water to produce 1 egg. Feeding too much treats, table scraps and scratch means the hens will eat less of the food they need, so only feed limited amounts of those. Overfeeding scraps and scratch can also cause the hens to get too fat. Overweight hens don't lay eggs.
Molting is a natural process which allows hens to replace old, worn feather by shedding them and growing new ones. It is usually triggered by day length (shorter days), but it can happen any time of the year. A stressful event can trigger it too. Most hens molt once a year, usually over winter in their second year and it is advisable to let to let your hens molt in their second year. Regrowing feathers takes us much of the body's resources as egg laying does, so during a molt most hens won't lay. A molt usually takes between 2 and 6 months to complete and unfortunately this process cannot be rushed. Some extra protein in their feed will help the hens a lot during this time.
Disease and/or Parasites
Diseases and parasite infestations will cause hens to either lay less or stop laying completely. Good parasite control is important and if a hen shows any sign of disease she should be isolated and treated asap. Some of the most common parasites that can cause a drop in egg production are mites, lice and fleas, which can be controlled by regularly dusting the hens, their coop and run with a good quality poultry dust. Internal parasites to look out for are round worms and tape worms. Deworm the flock every 6 months as a precaution. The withdrawal period for most dewormers are 14 days and the eggs laid during that period should be discarded and should not be incubated. The residue in the eggs causes severely handicapped chicks and most die shortly after hatching.
Diseases that influence egg production are fowl pox, coccidiosis, infectious bronchitis, Newcastle disease, Avian influenza, chronic respiratory infection, fowl cholera and infectious coryza. Most of these diseases can be controlled by vaccinating the flock and maintaining good hygiene practices.
Too high or low temperatures also affect egg laying. In winter aim to keep the temperature in your coop above 55*F and in summer make sure they have plenty water and shade to help them stay cool. Feed treats like watermelon, make sure the hens have plenty fresh, cool water to drink and put a fan in the coop if possible.
When hens go broody their hormones tell them to stop laying eggs and incubate and hatch them instead. They will sit in the nest box all day and night, refuse to get up and steal other hens' eggs if given the opportunity. Unwanted broodiness a nuisance, but there are techniques to "break" them and get them back into production.
The hen's age
Hens that won't lay can either be too young or too old.
The average age of a hen when she starts laying is 6 months. Some smaller breeds like Leghorns, Stars and Australorps lay sooner and larger breeds like Wyandottes, Plymouth Rocks and Orpingtons start later. The first 2 years of a hen's life is her most productive. By the time she's 5 years old she will only lay half as frequently as she did during her first 2 years. Good egg laying hens have 2 egg laying cycles of 50-60 weeks each. After that there will be a sharp decline in egg production.
A good sign of a pullet approaching lay is the colour of her comb. If her comb turns a bright red colour she's ready. The reason for the colour change is to show the rooster that she is ready to start laying fertile eggs. Her pelvis will be wider and if you look at her vent it will be moist and pink.
There is also a simple test you can do to check if your hen is laying/ready. Hold the hen firmly and turn her on her back. Put your fingers on her breast bone and work your way down to her vent area. You should feel 2 bones sticking up. These are her pelvic bones. If you can fit only 1 finger upright between her pelvic bones she's still roughly 4 weeks off laying, 1 and 1/2 fingers means she's a little closer, 2/3 weeks and 2 or more fingers means she's either close to or laying already.
Another reason you are not getting any eggs may be predators. Mice, rats, snakes and some other animals steal eggs, so make sure your coop and run is predator proof.
Free ranging hens sometimes lay their eggs in secrets nests. So your hens may be laying after all, but not where you want them too! If this is the case keep them cooped up for a few days so they will learn to use the nest boxes. Fake eggs or golf balls in the nest boxes will make them more attractive for hens too.
If you are not getting any eggs, but are sure your hens are laying, you might have an egg eater or two in your flock. Look for signs like yolk smeared on the nest box materials