Answer: I think it is suffering from shock due to the change in growing conditions from the grower to your indoor location -- and possibly underwatering as well as overly warm conditions.
I would strongly suggest you plant it outside on the east side of your house where it will receive full direct sun all morning. Sun all morning through noon is actually considered full sun, but it is not hot sun.
Since it has been inside for two weeks, you may need to recondition it to being outside. Set it outside in a shady spot and each day move it so it receives a bit more morning sun for a longer period gradually working up to all morning. After about a week, plant it.
It sounds like the soil ball has dried out and turned hard so water is running off instead of soaking into it, sort of like a dried out sponge. To counter that, fill a bucket with tepid water and hold the container submerged in the bucket to rehydrate it completely. When it is rewetted it will feel much heavier. Then allow all the excess water to drain out the holes in the bottom of the pot. After that, keep the soil just damp. When you water water slowly and make sure it is soaking into the soil and not running out the sides. If it is cool outside and it is in the shade you may not need to water it too often, but use your finger to check the soil and see.
When you plant it, plant it no deeper than it is growing in the container. Water it thoroughly at planting time to settle any air pockets and then mulch over the root area with two to three inches of organic mulch.
Water as needed to keep the soil evenly moist but not sopping wet or saturated, and not bone dry. To know if you need to water, dig into the soil under the mulch with your finger. If it is still damp, do not water yet.
Since it seems to be stressed, you may also want to consult with your retailer. Good luck with your Pieris, I hope it will be okay.
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