Honestly, those are the most difficult plants you have. Sometimes I've found that there are some plants that just do not do well in a tank, for no apparent reason. That may be the case. Try moving it to another high-tech tank or getting some new stems. If that doesn't work, then just get rid of it, you can't win em all.
My Golden Rule of planted tanks: WWTAD-
"What would Takashi Amano do?"
RAOK Club #69