I had a similar problem at my home, but only one of seven Arborvitaes died for two years in a row. With six survivors, I didn't want to give up on that one bare spot. So the last time I replaced the Arborvitae, I removed a bushel full of soil and replaced it. I thought perhaps something had been dumped there (automotive fluids from the neighbor's driveway perhaps) and the soil was damaged or toxic. Amazingly, that final soil overhaul worked and the newest Arborvitae has survived for four years now. It has even caught up in size with the rest in the row. I'm not sure if my story will help you or not. Sometimes, though, soil can pass a soil test and still have issues that plants cannot overcome. Maybe a major soil overhaul is in order. Also, you'll want to make sure that the water percolates down through the soil profile when you water so that the Arborvitae roots aren't getting too wet. You could also consider planting Red Cedar (Juniperus virginiana), but there are a couple of considerations. First, Red Cedar is hard to find. Forest Farm (www.forestfarm.com has them, but they're only available in a very small size). Second, after a number of years, the Red Cedar would need to be pruned like a hedge to keep it from going up 40 or 50 feet. It does respond well to pruning, but that's a lot more pruning work than you'd face if you can get the Arborvitae to succeed in the same spot. "