Q.Why is my Ilex crenata ‘Chesapeake’ sparse with yellowing leaves
Five plants along deck border are approximately three-four years in the ground and have never looked hardy. Instead of deep green, dense leaves, the plants have sparse and yellowing leaves and have failed to gain height. The soil in the bed is clay augmented with topsoil and peat. Have dusted with Sevin and have been watering regularly and deeply this spring. Expected to see an improvement but may actually be showing more yellow than before.

Certified GKH Gardening Expert
You mentioned having clay soil. Is the ground swampy? If the ground stays wet, especially with regular watering, the roots can't "breathe" and falling leaves or yellowing would be a sign of this. Too much water causes iron chlorosis as well and the soil may be lacking beneficial nutrients that the shrubs need to attain optimal health. I would recommend having the soil tested and correcting your clay soil accordingly. These articles will help too: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-yellow-leaves.htm