[Dr Winston O Boogie jr]

It sounds like the problem you have is with your soil and not with the grass itself. Even though all the advertisements for Zoysia say that it performs well in sandy soil, I've that to be not true. I have a few problem areas and I decided to try some Zoysia plugs. Most of them are doing very well, but in one area not so well. I noticed that the soil in that area is more sandy then the rest of the lawn.
Like you, I've been adding some topsoil. I've also started putting some topsoil in the holes before I put in the plugs. That seems to help a lot.
The problem with a lot of sand is not only does it not contain a lot of nutrients, but water goes straight through it so the plat doesn't get enough water. The topsoil will hold the water longer.