patches of dead grass

Hello

I had a question about grass conditions. I have only this year converted to fully organic lawn. I have been using Espoma Organic lawn food, because it has a decent amount of Nitrogen content. Anyways this year I have noticed some patches of dead grass. They are round to oval shaped and are only about 4 -5 inches in diameter. I do not believe this is due to moles or grubs...so I know it could be a hundred other things...any ideas. I live in the Chicagoland area and would prefer an organic solution.

What type of grass is it?
Did you rake the soil before planting (to eliminate the possibility of local soil imbalances)
Have you had extremes of weather recently?
Is there shade or sun in the spots at variance with the rest of the lawn?
If you plant again, does the grass die again in the same spots?

It was planted (sod) by the builder so I am assuming it is a mix. Also it has been getting warmer but nothing too dramatic. Lastly the spots are in direct sun as is much of the lawn...and I have not yet tried to replant in these areas.

sounds like a visit from a dog.