What TinyToon said plus make sure you're using some kind of rooting agent high in B12.

You need them to be in oxygenated water, I put a drop or two of superthrive in mine. I stay away from rockwool because I've had poor success using it for clones, I have an Aerogarden and when I use the sponge inserts that came with it I've had double the success and actually had roots within four days once, have never been able to get it to do that again so not sure what I did right that one time.

Personally(aside from knowing the strain/sex) I've found fresh seeds to be much more productive and grow much faster. I have seeds and clones started at the same time and there is no comparison in growth rate, 2 to 1 at least.