The most common problem that I see is that the grass is cut too short. Cutting too short in the Summer will definately cause the grass to decline and weeds and insects to move in by weakening the grass. Starting in Mid to Late June the lawn should be cut at at a minimum of 4 inches or the highest setting on the mower if using a push mower or ride on tractor.
This is a typical example of cutting too short
Also watering needs to be increased going in to the Summer especially on the hotter days that are in the 90's. If you have a sprinkler system you should be watering some time between 11PM and 5AM ideally.
When this happens to the turf then recovery will happen when the weather is cooler. Aeration and seeding in September will speed up the recovery.