I had the same problem, and for me it was actually that I had been using tiny seedling fabric pots and they were drying out too fast. I was probably watering too little too often (about once every 1.5 days, sometimes only misting), but the problem seems to have been moisture retaining. I took the worse curling one and repotted carefully into a plastic pot, watered until a little runoff, then left it overnight and it bounced back like a newly born. In your case, I am venturing to guess that your soil too is drying out too fast - but watering often is also bad, so somehow, you need to keep it moist; what's your humidity level? I was also at 40% so no wonder they were drying. (picture shows how it bounced back after repotting and a good soak. I was using a couple of small LED bulbs, and switched to
mars hydro 600W at 20 inches. so I think your light is probably fine.
check out my question for more photos.