Lawns are not only about mowed grasses and fertilizers; they have other problems also like yellow grass, insects, diseases in grass and other animals which make burrows. These can destroy your whole hard work in few days. I suggest that you hire a professional gardener if you are facing any of these problems.
Apart from using fertilizers also use fungicides to prevent fungi like mushrooms which grow in the shady and moist area of the laws. Use insecticides to prevent the damage from insects. The problem with insecticide is that insects develop resistance to them very quickly so you have to be in continuous touch with the local agency and keep alternating insecticide to ensure proper effect.
There are other diseases which affect the grass they mostly occur if the lawn is very moist. So you can see that more than enough water is also not good.
One more thing which can destroy the lawn overnight is the burrowing animals. They can dig up your lawn overnight and you will get up to a hole in the ground. Be careful of the anthills also and literally nip them in the bud.