It could be insects or diseases. Cut out a small piece of sod (2" depth) using a shovel and flip it over. If you find 5 or more white grubs (chafer or billbugs) or leather jackets (crane fly) per sq ft the issue is insect related...

Protecting lawns against European chafer (diggermagazine.com)

Managing Crane Fly in Lawns | OSU Extension Service (oregonstate.edu)

This time of year, disease become a problem if you have not fertilized or starting irrigating. Fertilize your lawn with a pound of N per 1,000 sq ft in June if you have not already. Start applying water 3 times per week at 1/4" per application. This should alleviate any disease pressure.