Hi, I had the same problems, yes, I figured this out. Three things:
1. Llimnophila aromatica is extremely sensitive to dGH and dKH and espessially to their changes, she loves both dKH and dGH very low. I keep three tanks:
The first one, where this Limnopyla is flourishing has pH=6-6.3, dKH=1.5 and dGH=2-2.5.
The second one, where it is surviving very well has pH=6.2-6.4, dKH=2 and dGH=3-3.5.
The third one, where it loses tops and dies pH=6.4-6.5, dKH=3 and dGH = 5-5.5.
Moreover, if I adjust dKH (adding KHCO3 or dGH (adding Ca of Md salts) in the first two tanks by as little as 0.5 degree, my Limnophila looses tops on the next day (they became brown and look unhealthy, so I remove them), but becoming healthy again in 4-5 days.
2. Llimnophila aromatica is extremely sensitive to the nitrogen amount in your water. You mist keep nitrates at 8-10 ppm N, once nitrogen goes above 20ppm, it looses tops and eventually dies.
3. Llimnophila aromatica is extremely sensitive to the phosphorus amount in your water and she needs a lot of phosphotus. You must keep phosphates at 2ppm, you must never allow them to fall to 1ppm or less.
IMPORTANT NOTE: I have heard (from quite reputable sources) that if you ROOT FEED Limnophila, it is much less sensitive. I have NOT tried it, because I avoid root tabs: they increase pH (by definition, they are made of CaSO4 with small amounts of microelement additions) and I keep many very sensitive plants, I want to be able to fully control my pH by changing CO2 amount and/or dKH myself.
Thanks for the tips. I'm not sure what my gh and kh is. What ever the equivalent of around 40 ppm would be. I do feed them with root tabs. I find it odd that out of about 30 stems maybe five of the are like that.
I've decided that I'm only going to give the take natural sun light on Sunday and Wednesday, fertilizer day.