I have tried another new location for the Chamaedorea costaricana seedlings in March, but they still look no healthier. I'm really not sure of the cause of their poor health, the symptoms imply too much light, but even in very heavy shade in the low light levels of December the leaves were burning, whereas the other Chamaedoreas in the same area were looking stretched, suggesting that there was insufficient light. Either way, I think these seedlings are probably past the point of recovery now. I will keep nursing them for a while longer, but I'm past expecting any improvement.