It sounds like you did a good job planting your arbs. I suspect that the culprit is the hot, dry summer and fall we’re having. Newly planted shrubs need to be kept constantly moist during the growing season. In your sandy soil, this would have meant a thorough soaking every other day. To help them at this point, you shuld try pouring Mir-Acid (a food you dilute in water) from a watering can over the foliage. Mir-Acid can be absorbed directly through the foliage to feed the plant instantly. If you don’t have Mir-Acid then regular Miracle-Gro will also work. Also soak the soil by placing a slow running hose at the base of each plant for an hour or so to give the water a chance to get deep into the soils and the arborvitae’s roots. There’s rain coming that will help but deep soaking will take more than an inch or two of natural rain. This watering is especially important late in the growing season as plants (especially evergreens) are trying to store the moisture and nutrients they need to make it through the winter ahead.