This slows the rate at which water and air move through the soil and acts as a barrier to root growth. Alleviation of this problem is accomplished through aeration or by physically disrupting the compacted layer with a plow. To check for a compacted layer, insert a knife or screwdriver blade 6 inches into the soil. If significant resistance is felt, you probably have a compacted layer. Lawns thin out for several other reasons.
Drought stress thins out the desirable species, and disease pressure causes the lawn to thin.
Insects will kill the turf. After the turfgrass is gone, weeds move in. The presence of a great number of weeds is a sign the turfgrass is not thriving. Before you can make the lawn succeed, you need to find the cause for its decline. Weed growth is a result, not a cause.