dont give up. i had the same issue. but now in the same setup but with more CO2, i have r. macrandra variegated doing quite nicely, and r. macrandra green doing well too.
try planting a stem of it right between your foreground and midground. it wont look nice, but thats where the water is least likely to stagnant IME, and thus CO2 should be the most available as fresh CO2 rich water from the diffuser constantly washes over the leaves. if it does fine there, then you know what your issue is.
I'm not giving up on it yet. Lets just say I'm moving on. I had my rotala magenta in the midground of the tank and I could see the bubbles blowing all around the plant. It was doing alright for the first 2 months then it started stunting on new growth. The problem is I have been trying a few different methods of co2 diffusion over the past few months and during my experimentation there was some times the co2 wasn't ideal. I'm sure I will try it again sometime. It is a beautiful plant