would check the temp of the rockwool you want it at about 80-85 degrees if its to warm like over 90 it will cause problems.
one other common mistake is to leave the rockwool cubes setting in water in the tray, rockwool holds the perfect amount of air and water as long as its not setting in water.
theres not much to cloning if the above things is ok and you presoaked the cubes in 5.5 ph water and use some form of rooting hormone or solution you should see roots coming out of the cubes within a week or 2 and some of the leaves may wilt and turn brown but generally not until after they are rooted and you are removing the dome gradually over a few days to harden them off.
you might also put your light on 18/6 if you have it on all the time as it generally allows them to root quicker by giving them a dark period.