It may be too hot for your tree. Or the tree might not like the soil inwhich it is planted. 6 to 7 hours of direct sun is very hard on some cultivars of Japanese maples. If it were my tree, I would immediatly provide it with shade. It sounds like your tree is to far gone to expect it to pull through if you now dig it up and replant it somewhere with less sun. Japanese Maples do best planted on the north side of a tree or sturcture that is larger than the maple and also where the west sun is fully blocked from 12.30pm through sunset.

How tall is the tree? Is it even possible for you to provide it with that kind of shade where it is currently planted?

Next, after doing a light-scratch bark test to see if the trunk is still green at least half way up the tree, and if you find that the tree is still alive; then, the next thing I would consider is the drainage of the soil and also the type of soil in which the tree is planted. If your soil is slow draining the roots may be staying too wet and having trouble taking up its needed soil derived elements. If the soil is very sandy and drains to quickly it may be lacking both constantly moist but not soggy soil with adequate humus. If you find the latter to be true, hopefully you have a quality local nurseryman whom you trust that you could ask advice on how to suppliment the bed in which the tree is planted.

Years ago, I tried to plant a Japanese maple on the east side of my house and it did not survive. Too much direct morning sun, and heat that the side of the house absorbed from that morning sun. The tree got too much direct sun and too much radiant heat long after the direct morning sun was gone.

Currently I have a Japanese maple that a nurseryman is keeping for me until this fall. When he sold me the tree back in early march of this year it was his last one of that cultivar. It is also a cultivar that is difficult to find. When he first showed me the tree, he realized that it was too loose in the pot, which meant the roots were not growing in the potting soil correctly. Otherwise the tree looked fine. Since I really wanted the tree and did not want to plant it until it went dormant this fall, the nurseryman offered to replant it in a possibly better working potting soil that he had custom mixed. He also offered to keep the tree and care for it until early September. At that time I will pick it up and he has agree that if the tree has not done better and still is not growing properly I will not have to pay for the tree.

The point of this last bit of information is to let you see how even some experienced nurserymen also have trouble getting it right when it comes to doing, from the git-go, exactly what is needed for some Japanese Maples.