It is normal to turn brown where irrigation coverage is lacking. Kentucky bluegrass (KBG) lawns may require 2.5 inches of water (or more) per week during the heat of summer.

Also, assuming you have an automatic sprinkler system, please consider testing it to determine the exact amount of water that is being applied in the areas of concern. Even if it is getting less than 0.1 inch than other areas, that difference in water adds up quickly under hot summer growing conditions.

One easy way to test the system is to place identical small cans or bowls (like cat food cans or tuna cans) in areas where the turf is healthy and green, as well as areas where the turf is brown. Run the sprinkler for 10 minutes, then compare the amount of water in each of the cans. Also verify that heads are working properly, that they are properly aligned or not too deep, and nozzles and filters are clear.

You can also stick a long-bladed screwdriver into the problem area, then compare the ease of blade entry with that of another area that is green. If it is more difficult to insert the screwdriver into the problem area, then it's possible that the soil is compacted and water may be running off of it before soaking in. The solution to this is to make sure to core aerate twice each year; once in the spring (April) and again in the fall (September). Multiple passes need to be done for this to be effective, especially in the problem areas.

After aeration, you can also overseed with a high-quality blend of drought tolerant KBG. Seeds will fall into the aeration holes, and by watering new seed twice per day, it will germinate in a couple of weeks. There are several varieties of KBG that are noted to be drought tolerant, or there is also Hybrid Bluegrass (Thermal Blue, Spitfire and others) that is very similar in appearance but has better heat and drought tolerance.

Dr. Tony Koski, CSU Extension Turf Specialist, has written many blogs on these lawn issues:

http://csuhort.blogspot.com/2016/07/turf-rant-believe-me-it-is-irrigation.html

http://csuhort.blogspot.com/2015/07/what-pain-in-ascochyta.html

http://csuhort.blogspot.com/2016/06/its-ascochyta-time.html

http://csuhort.blogspot.com/2017/05/ascochyta-leaf-blight-on-lawns-2017.html

And for more information on lawn care, see these links:

http://extension.colostate.edu/topic-areas/yard-garden/watering-established-lawns-7-199/

http://extension.colostate.edu/topic-areas/yard-garden/operating-and-maintaining-a-home-irrigation-s...

http://extension.colostate.edu/topic-areas/yard-garden/lawn-care-7-202/

http://extension.colostate.edu/topic-areas/yard-garden/irrigation-inspecting-and-correcting-turf-irr...