Why Are My Rhododendrons Dying?

We built a new home in the country 2 years ago. My vision was to plant rhododendrons in the wooded area after clearing all the brush from the woods floor. I have planted 5 plants – 2 from one nursery, 1 from another nursery and now 2 more from yet another nursery. This is in Southern Indiana . The first two bloomed beautifully this spring, had new growth, growing nicely and suddenly all the leaves drooped and have since died off. The 3rd one was a beautiful smaller plant and did not bloom, but grew steadily until just a few weeks ago. It is now doing the same thing. The last two look OK, but I am at a loss of what could have gone awry.

I read an article in the Better Homes and Garden magazine about a chemist in Chicago who grows Rhododendrons and has worked tirelessly to find a perfect soil for them. It was 2 parts pine bark mulch, 2 parts top soil, 1 part sand. I used this “magic” soil for all of the first 3 and they seemed to be thriving with a sudden death. This is getting expensive and very frustrating.

Do you have any ideas?