first of all, are you pre-soaking your rockwool cubes in 5.5 Ph water for a few hours before putting your seed in? you also might want to try germinating using the paper towel method and dropping the seed in after it gets its taproot. as far as your question about the drip system, the expanded clay wicks water to the rockwool, keeping it from being soaked. you can also have your rockwool be too wet as well as too dry, so you have to find a happy "medium" somewhere. (get it?)
