So, I built a multi tier planter box, used sand as a base till about 7 inches from the top, but after a year the base is either compacting or running off to a side...either way, the center has dropped 6 inches and the next tier 3.

It is fully planted, with a variety of dwarf conifers. My question is: is there a way to raise the trees back up? How do you add more base material in a situation like this? Or do I need to simplify the structure/remove levels at this point?

the majority of the trees are doing well, I’d hate to risk damaging them by transplanting them at this point