to NGA ...
Daisy is right to suggest that you check the ph in your soil.
I'd like to add a couple of thoughts of my own. I have a pink dogwood tree, 'Cherokee Brave' that the previous owner of my home sited poorly. It is growing in lousy soil and it just keeps on going. What I am saying is, that with this tree, I didn't have to have perfect soil or conditions. From what I have read, they are pretty adaptable.
As for digging up your tree, I would screech out a very loud, "DON'T DO THAT YET". Your tree is been in the ground long enough to have an established root system.
One of my own rules of thumb is not to mess with the roots of a stressed or non-performing plant until I've tried everything else.
Putting compost in a planting hole is a total waste of effort. The feeder roots of most plants are near the surface. The deeper anchor roots serve to anchor the plant in place and to pump up moisture from deeper levels. They don't feed the tree. Also, when the compost decomposes, the tree will sink ...
It is wiser to follow what we see in nature. Nature dumps all leaves and other debris on top of the soil. It's worms and other critters that pull the nutrients from decomposing organic materials down deeper. So, I put everything on top of the soil and avoid messin' with the roots.
Also there are dogwoods all the place around here. The woods and the drive through the canyon is full them.
My guess is that if you look at the dogwoods in the woods, you would see a lot of leaf litter around them ...
That's Nature's way of feeding the soil.
As a first step, I would put down mulch of any kind under the tree. Make sure the mulch doesn't touch the truck of the tree. The mulch will feed the soil and activate the soil food web that will enhance root growth.
I don't know what you have been feeding your tree with over the years. You can keep it simple by putting down some steer, or even better horse, manure. This too, will feed the soil food web. Or you can use recommended fertilizers for trees. (I have never had to take that step, so I cannot recommend anything.)