I have two Snow-in-Summer (Cerastium tomentosum) plantings within 2 feet of each other and as you can see in the picture, their health seems to be quite different. Any thoughts on this?

Of the few notable differences that I know of, the one on the right may get more water funneled to it by the way the landscaping runs. The one on the left gets slightly longer morning shade from the barrel half that it is planted next to. However, I would say that both get "full sun" overall with some late afternoon shade.