The problem might be that they are not getting enough light. Rockwool is a great product for growing plants and it has been used for centuries. It's made from ground-up volcanic rock so it provides nutrients and structure for plants to grow. It also helps maintain moisture in the growing medium. There are many different types of rockwool, each with its own unique properties.
Rockwool is like a sponge and absorbs all moisture from the air. It's great for storing seeds but if you put them in it they won't get a chance to germinate. You need to keep your seed starting containers dry and make sure you water the soil before you cover the seeds with the rockwool. I would use vermiculite for a seed starting medium because it's a bit drier than the rockwool.