I have come across a few lawns this year that have had 2ft sq to 3 ft sq very dry to nearly dead grass in those areas. What causes this when the rest of the lawn is doing great with no problems? What I am finding is that when I probe the ground in those spots there are large rocks 3 to 4 inches under the soil. Why do those areas start stressing out? With all the rain we received early on in March and then not a lot of rain after,  what happened was those areas dried up faster because the root system of the grass sitting on top of the rock were only 3 to 4 inches thick. This caused the soil to dry up much faster and cannot hold as much water as other parts of the lawn.  What can you do to fix this problem? You need to dig up the rock.