Answer: It sounds to me as though you've done everything humanly possible to have a healthy lawn. Because I can't personally inspect the lawn, I can suggest the next best thing: take a sample of the problem into your local cooperative extension office. Helpful folks there can diagnose the problem first hand and make treatment recommendations. Judging by your zip code, I think you are in Adams County. If so, here's the contact information: Colorado State University, Adams County Cooperative Extension, 9755 Henderson Road, Brighton, 80601-8114 (303) 637-8100
Best wishes with your lawn.
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