Dark limp sprouts

I germinated four seeds in rockwool starter cubes. I placed the four cubes on a plate under a cfl with a space heater about four feet away from them. I watered the rockwool when it felt dry. A few days later I noticed that three of the seeds had sprouted so I put them into soil under my 400w MH. Once again I watered when the wool felt dry. Yesterday two of the sprouts had pushed through the wool but they were dark and limp. Ive seen healthy sprouts before so I assumed these were dead and pulled them out. Anyone have any idea why this might have happened? This is my first time using rockwool so I might have over watered...let me know what you think.