Rhododendron Problem

About 4 years ago I planted 2 rhododendrons in very large pots. Both planted in acid soil. One plant is in semi shade the other is in full shade. (although,obviously, they could be moved to different locations) Both plants initially flourished & grew well for 3 years. This year the one in semi shade is showing signs of distress the leaves have turned yellow & the plant is wilting. The other plant is thriving double the size of the one in semi shade, glossy leaves etc. I have a suspicion they are not the same species of rhododendron but cannot verify this. They have both been fed etc. in exactly the same way. Any ideas??????????