Please ignore this comment if you have had a satisfactory reply many years ago!
I was just scrolling through the photos and comments regarding rhodos when I saw your (this) comment.
You mentioned that your rhododendron was close to the underground masonry. IMHO that might be what the problem was: Most masonry is alkaline (having lime or calcium ingredients) and is the enemy of rhodos and azaleas. We have had many over the years and some closer to sidewalks etc. than I'd like, but as long as we gave it rhododendron fertilizer every year, they managed to survive.
As an aside comment: I recently read about a fellow in the UK who was successfully growing rhodos in an old limestone quarry (his backyard!) and to make a long story short, he discovered that the element Manganese (Mn), when added to the soil (I think he said he found coffee grounds had more than normal Mn and used them to supplant his naturally alkaline soil) allowed his rhodos to grow & flourish.
Cheers, Barry M.
