My wife planted some aptenia sprigs in a circular flowerbed. The bed is a circular brick wall, making an outer ring around a short, inner brick wall in the middle containing some flowers. The aptenia spread and eventually filled the outer ring of the flower bed. A few months later it started thinning out in the soil and started growing more on the rock edges of the outer brick and inner brick. The plant on the brick looks healthy, but on the soil, there is just a meshwork of roots or stems or whatever - no greenery at all. It looks like the green is just avoiding the soil. What would cause this, and how would I fix it?

The attached photo is from a couple months ago, where you can see the thinning starting. I've added a red line to accentuate the brick rings.

Thanks!