This can be caused by compaction. Compaction that may be left over from the installation of the lawn or by compaction that is caused by traffic on the lawn. The solution for compaction is aerating the lawn to open it up. This can be done by a professional service or by using a hose end aerator yourself.

The other main reason is irrigation problems. Next door to me I am seeing a problem caused by a rotor sprinkler that doesn't sweep across the entire lawn leaving part watered by only one sprinkler. I have personally had problems with sticking sprinklers that don't pop up and by clogged sprinklers. If you don't run your irrigation system occasionally when you can see it, you don't know.

The other reason can be reduced water pressure or poor sprinkler design. This you have to check by using the pie pan method. This means running the system with pans around in the bad areas and good areas and comparing how much each is getting. A rule of thumb is the difference should not be more than one third.

If it is, you may need someone look over your system and see what needs to be done. Adding an additional sprinkler seems an easy solution, but because it can result in reduced pressure to your existing sprinklers it can just make the problem worse.

In any case the short term solution is using a hose end sprinkler to fill the need until you can get the right solution. Do Not just increase the time until even the dry areas get enough. We can not afford to waste water that way.