Answer: It sounds like you have taken good care of your lawn. Sometimes problems such as this are related not only to fertility but also to pH, compaction, the variety of grass seed, and other underlying causes. I would suggest you begin by running some basic soil tests to check the true fertility needs and also the pH to see if you need to lime. Do that, along with a top dressing of good quality compost to add organic matter. You can apply these at the same time if you wish.
At some point you might also want to consider doing a core aeration (not the spikes, this pulls up up cores or plugs of soil) to open the ground to moisture, air and nutrients.
You could also consult with your local county extension to see if they have suggestions as to the best suited grass variety to seed, based on your local conditions and the soil in your yard.
Make sure you are mowing high enough, two and half to three inches, in summer. This helps the lawn stay healthy. Also mow often enough that you are not removing more than one third the grass height at a time. This can mean more than once a week in the spring.
Your local county extension should be able to help you with the testing and interpreting the results, as well as with troubleshooting. In the meantime, their web site offers quite a bit of lawn information you may find interesting.
I have been surprised to see some changes in generally recommended lawn practices based on research over the past ten years or so, you may be surprised, too.
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