shade or fungus or other?

so i have these thin brown spots in my lawn (kbg sod last year). dont think it's a watering issue because it's actually about 10 ft from the sprinkler head and the grass around it is fine but a bit thin.

you can see that it does get shade from that big oak (i'm looking north in the pic) the lawn is pretty thin back there but i overseeded with some fescue to see what happens next year.

i took pics starting from distance to close up

any thoughts?