What PlantManBee was saying is right, you must prepare your Rockwool, the manufacturering process of Rockwool has a lot of lime in it.
They say its prewashed but in my experience with Gordons stuff it isn't enough, if you fill a bucket with pH balanced water and let your cubes soak until they sink then collect the run off of the cube in a saucer or something and pH test the water, the reading will be around 8ish.
I do what PlantManBee was saying but then I soak them again and test again, repeat until run off is 5.8, I'll then leave them in the grow area over night before dropping pregerminated seeds into them, the flicking out of water is the most important thing too do.
I love Rockwool, I've never had good seeds not break the surface, it holds a good amount of moisture, you don't get damping off if your clean, it's easy to deal with, plants grow fast in it and it's so easy to then move it into what ever system your running.
I'd say your Rockwool was just to damp, that's something else to be aware of, its very easy to overwater seedlings in Rockwool, you can let it get real dry before watering again.
Sorry to give you the full "How to use Rockwool tutorial", been smoking some good sativa and I really like Rockwool.
Best of luck with your grow !