From a fellow grower: I’m trying to figure out what went wrong. I dropped five seeds in a cup of water that had been pH to 6.2. I let the seeds soak for 24 hours then I poured them onto a paper towel and place them in a ziplock bag. I have been using this method for years and years. All five seeds germinated and grew a tap root about 1/2 inch to 3/4 inch long (1.5-2 cm). I used a sterilized set of tweezers to place each seed in a 2x2 rockwool cube and placed them in a germination tray with a clear lid. Then I placed the tray in my grow room which is set at 75° Fahrenheit with 70% humidity, under a MarsHydro FC -E3000 dimmed down to 50%. After 4 days of watching them closely only one seed ever started growing to an actual plant. I cut the other four rockwool cubes open trying to figure out what happened. 2 of the tap roots had turned to mush and the other 2 look exactly like they did when I put them in the cubes.
Most likely damping off due to overwatering, which is the single most common cause of seedling failure.
Agree with @MidwestGuy. Rock wool holds a lot of water. Try a solo cup with cannabis friendly medium for the next grow 
I get good results with Rapid Rooters (rockwool). To use them correctly I immerse in tap water then squeeze out almost all of the water: you just want damp. Then plant sprouts, cover with a dome and mist the dome with distilled water under a 24/0 light schedule. A heating mat can help too.
I’m in the plant into soil group. I don’t even soak my seeds. I just moisten the soil, and plant the seed directly in the soil, and put a baggie over the cup. Works pretty much every time.
Aren’t rapid rooters just peat plugs?
It sure does. I use rockwool exactly for that reason. Soak in pH 5.8 water for an hour, set on a dish drying rack for 15 minutes for some excess water to drain off, then dome and leave alone. I add maybe 5 mL of water to each cube once every 2 days. They don’t take much if you leave em covered.
They’re rock wool.
“RapidRooter’s breakthrough technology produces a unique matrix of Peat moss and binders. With RapidRooter, you have the optimal air-to-water ratio…” I feel like I’m missing something.
Well that’s what we called 'em. But they’re rock wool. Stoner science I guess.
yep that makes perfect sense. I feel that in my bones.
Are you saying lights on for 24 hrs or lights off for 24 hrs?
Lights on 24/0
Generally autoflowers only need 18/6 lighting for the entire grow.