I agree with all the statements about the need for additional fertilizer.
This could be the problem.
But I also have a gut instinct that this stand of turf may be missing a little of the notorious seventeenth nutrient : OXYGEN!
I see this happen sometimes with lawns that get 'babied' too much with water, especially after everything is up and already growing well, and it is over watered.
In many cases the nutrients are there, but there's not enough open pore space left from all of the water saturation.
Yes...I know it's on a slope... but slopes can be soaked too much too...