Struggling akeake

Hi there!
Would be grateful for some advice regarding my less-than-thriving akeake, and a few other trees!

Last winter I planted a range of native trees and flaxes along a sunny west facing fence line in Lower Hutt, many of which don’t seem to be doing well, particularly the akeake which has sparse brittle leaves. I had wondered whether the soil was too dry as there is a sharp dip behind the fence line where I think some of the water (and soil!) runs down, as well as my mapou and muehlenbeckia astonii showing yellowing leaves. However, I read that akeake tends to thrive in dry, poor soils! Any clue as to what may be happening and how to remedy it? The soil looked in poor condition when we moved into the property but we dug through plenty of organic material a good 6 months before planting and did some basic sheet mulching. The same types of trees which we planted in the front yard at the same time are thriving, so I don’t think it’s anything to do with climate.

Thanks so much!