Soil structure, texture, fertility and drainage can vary significantly from area to area or spot to spot in a garden. Even within a few feet of each other. Without examining the area first hand, it is almost impossible to offer meaningful advice or suggestions, provided you have give the plants the care they needed.

Take a soil sample to your local extension office for testing.

First, like many extension services, OSU no longer does soil tests. You need to prepare a sample and send to the independent testing lab of your choice.

Second, a soil test is not going to supply all the info you need. They deal almost exclusively with soil fertility/nutrient content. Drainage, structure and texture are not addressed.

I can think of various reasons the rhody might have failed, not so many for the box. Of concern would be the possibility they were planted too deeply, the rootballs were not adequately disturbed to encourage proper development and establishment and, despite your best efforts, insufficient irrigation during the period of establishment.