What is the north side like - is it well-drained, and how much shade is "more shade?" How many hours of direct sun a day? Was the lawn treated with chemicals before? How long as the lawn been in place - might there have been debris or chemicals left on that are from building, etc?
I don't pay much attention to my turf, whatever is there gets mowed when it needs it, and a lot of it in front is St. Augustine
that grew from next door. I don't claim grass expertise, but when dealing with fungal problems some of the things like straight 3% hydrogen peroxide (grocery store strength) sprayed on the spot might help, or some of the plant wash products. If you find a mix of too much shade and perhaps poor drainage are contributing to the poor health of the lawn you may need to decide what to do about the shade or switch the groundcover in the area. Your best bet for whatever you grow there is to be sure the soil is well-drained, aerated and healthy.Here is an old newsletter about brownpatch
and if you look through the library you'll find a variety of grass fungal problems, like dollar spot
and take-all patch