Are you watering right at the trunk so the rootball gets a good soaking? Have you dug down into the rootball to check?

tj

We checked a few inches into the soil to make sure it's moist, but after reading around here a bit I think we might need to let some water trickle onto it for a while.

Tree is severely diminished. Japanese maples do not tolerate wet soil. Dig it up and re-plant on a mound of good topsoil, bought for the purpose. Even then existing shriveling of some twigs may indicate it has picked up a pathogen such as Verticillium wilt and may continue to dwindle away*. Despite having been rescued from the sump-like condition you may have created by amending the back-fill of a planting hole dug out of heavy clay.

*May have arrived from vendor in a diseased condition - awful large pruning cuts visible on trunk and general very poor condition already apparent only a month after arrival tell me you were not supplied with a quality specimen from a top flight operation

Oh boy... no wonder they don't offer any type of guarantee. Hopefully the replanting helps.

I donno, I paid the nursery to have it planted the first time. It wasn't until it started dying that I tried to fix the swampy clay mess underneath it.

If the nursery planted it (and you paid them to do so) then they did a piss-poor job of t!! It should be common knowledge for any nursery worth their salt how to properly plant the trees they sell. Amending individual planting holes has long been an outdated practice and amending an individual planting hole in a poorly draining clay soil should be just matter of common sense. As cearbhaill stated, all they did was create a bathtub. Since they are primarily responsible for the tree's decline, they should replace it, guarantee or not.

Although I do agree that the tree was in poor condition to begin with.

Yes, the Nursery should have known better. It's a serious red flag if they don't guarantee their stock or work.

I would be in there (at their busiest time) ripping the MGR a new one!

I thought JMs need partial shade. Looks like it is planted in full sun. Maybe sun exposure has nothing to do with this situation.. I def could be wrong

Most JM's would prefer partial shade....at least afternoon shade or protection from the most intense portion of the day's sun. But most don't really need it and many cultivars will do perfectly well in full sun, provided they receive adequate irrigation.

Bloodgood is considered to be one of the most sun tolerant varieties, although it will not necessarily retain its color intensity in full all-day sun.

6a might be St Louis or similar in the Midwest. My Japanese maples crisp in full sun. If I were planting any for ornamental foliage I'd really make sure they got some shade.

The tree above looks worse than any of mine have. It could be from being taken out of a part shade situation in a nursery and being thrown into full sun, with flooded roots I suppose.

Also, trying to extrapolate from the picture I'd say the OP is in a fairly new neighborhood. My buddies who bought newly built neighborhood houses have way better grading and storm water flow compared to my 70 year old one off house. However thanks to construction techniques they have compacted terrible soil with the good stuff from the top sold off. Much tougher to grow trees there than in my yard.

What to do? Dig into the rootball and see how wet it is at different times. Modify your watering accordingly.