Answer: The most common cause of brown spots in lawn is dog urine. The salts essentially render the soil sterile so brown patches can remain, even if you reseed. I'd scoop out an inch or so of soil along with the dead grass, replace the soil with some from another part of the garden and then reseed the areas. To minimize the damage from dog urine you can hose the area off asap after the fact.
Best wishes with your lawn.
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