All the reply posters are right on! Are you using any nutes on the seedlings? Did you test the rockwool for ph before you put seeds in the cube?

I use the rockwool cubes too. I have found they tend to hold the nutes and burn your tender lil' ones. Don't take much with a seedling. It only has one root to damage and it's all over for that seedling. Just a couple of questions that come to mind worth mentioning.

Peace!
Ga.