It is quite likely the sod was lain on a medium no more plant friendly than the brick or stucco that encases your home. The roots probably penetrate no more than three inches of the soil profile, because neither water nor oxygen can be found much deeper. This will relieve some of that compaction, thereby encouraging more root growth. Do it once in the spring and once in the fall. Fertilize in the fall with a quick-release nitrogen fertilizer after your last mowing but before the grass turns brown.
Kentucky bluegrass uses between four and six pounds of nitrogen per thousand square feet in a year.
Fertilizer at this time goes into root production, not top growth. It will deplete roots reserves and create an unhealthy lawn. I know this is a lot to digest. Please call your local extension office if you have any questions. We must work together to make our Kentucky bluegrass lawns the healthiest and least consumptive they can be.