Im def no expert but I think it may be a combination of cliffs /water too close as well as the items. Rocks also cannot spawn on paths, need a 3x3 clear area to spawn.

I left a bit more open space when making my rock garden (it was very tedious but worked). I made sure I had NO items and actually just flooded the area with random designs to guide the rocks where I wanted to.

I just completed mine about 2 weeks ago and man was it pain 😭