Planting on top of retaining wall

First time posting! We are finally getting around to renovating the garden in our new home. The previous owners put lots of gravel on top of the retaining wall and it looks like clay soil. I love natives and tried planting some myoporum on top of the wall, but it ain’t doing too well 😂 Would improving the soil condition potentially help them grow, or is it just too hot there? Any other suggestions for the top of the retaining wall/inside the crevices? Based in Brisbane, wall gets full sun (north facing). Any advice would be greatly appreciated 🙏 https://imgur.com/a/r1UrcCY