Spot on.I don't think the media would be too much of a problem considering how the other three are doing alright. If all four of them started complaining then I think I would have to do something about the media.
I have some plants in mixes which contain generous amounts of lava rock, and none of them have ever shown any symptoms like that. As I and other said, your media may also be getting too hot inside for the roots. I know for a fact northianas in situ get subjected to temps above 100, and they certainly survive, but they don't look good. I'd try to cool the roots somehow and maybe give a weak dose of fertilizer.