The N.burkei is more troublesome. It almost sounds as though it has
root problems (droopy yellow leaves can be an indication or root rot).
This is probably a combination of the dry conditions it came from to
the wet condition it is in now. I'll assume that it is planted in
long fiber sphagnum. This being the case, you should make sure that
the plant is _never_ waterlogged, simply damp. Then you'll need to
strength the root system with either Ortho's B-1 mix or Superthrive.
You might also want to check the acidity of the soil (with a simple
acidity meter). You'll want the pH between 5 and 7. I have seen the
pH of growing go between 0 and 3 when root rot was involved... I have
no idea why, it seems to be some chaotic loop.
Chris