When plants seem to fail in a location, the reason might never be clear. Soil testing is a general panel, or specific things can be tested. But when you don’t have an idea of what to test for, it’s a challenge.
Please provide a photo or two from further away, maybe a couple of angles.
Guesses are things like pipe breaks: Irrigation? Water supply? Sewer? Gutter drains?
What might be buried there? Was a big stump from a prior tree left in place and buried with a few feet of soil? Any clues from construction of the retaining wall and patio? How deep can you probe and find nothing? Are any utilities located here?
Possibly the photos will suggest other lines of inquiry.

Hi, Jacki: I've attached a few more pictures further away, as you requested. Let me know if you can't open any of them.

I please attach directly if I can’t get them pulled in via Google drive.

Hi, Jacki:

I was able to see the photos. Thanks.
No clear answers that I can see. Some thoughts:
The other trees and shrubs are quite established, and roots reach much further than the plant canopy “drip line”, so root competition might be an issue. Your location is a bit of a raised bed, with concrete on three sides and a big tree & shrubs on the other. I don’t know of any way to solve this except grow smaller compatible plants.
Failure to establish roots into native soil is a primary reason new trees fail. This can be due to a number of reasons. Sometimes the tree is planted too deeply. Roots need access to air, and grow in top few inches of soil.
Sometimes the planting hole was amended with something. Only native soil should be put back with the tree roots. This is because water doesn’t flow well between different soil textures, so roots don’t, either. This can mean roots grow in circles, eventually girdling the tree. It can also mean you water the root ball but water isn’t reaching beyond the planting hole, or you are watering the area, but water isn’t moving into the rootball. You need to water and then check the soil to see that you are getting the amount of water to the right places. This often means checking with a trowel, your fingers, or a screwdriver (it goes into wet soil easily, and won’t into dry).
Check the trunk, is it healthy? I see some checking, but can’t tell if it’s an issue.
Planting on a slope is tricky, and you didn’t watch how it was done. Perhaps the hole wasn’t wide, which is another reason roots struggle to reach out. A slope is also a challenge to water well. A “moat” around the roots of a new tree is recommended to allow water to soak in well. It’s hard to do on a slope. I suppose if it was my landscape, I would make a bit of a terrace and plant the smallest tree I could. The terrace would hold water a bit, and a small tree would have time to grow roots into the surrounding roots before it got too big and required more soil.
There might be utilities that make the soil there extra-shallow. I’m glad you are asking about that.
Here is EC 1438, OSU’s basic new tree guide, referencing planting holes and watering concepts.
I’ve seen privacy screens and sun sails suggested for use while new trees or shrubs fill in. A planter box with trellis for a vine can be used also.
I’m sorry I can’t offer a more conclusive diagnosis.