I find that most of the trouble I have with clones is TO MUCH WATER in the soil, which will drown the little girls. I do not use a regular humidity dome, I just invert quart freezer bags over the individual pots with a little spray of water inside the bag to keep the humidity up.

In this way each plant will 'tell' me when it is time to remove the bags and expose them to the air. when I see signs of growth (roots starting to get a good start) I remove the bags. Usually a week or two into the process.
Also I do not put them under MH or HPS lamps until they really start to grow.
I leave them under cfl's as I have had experiences in the past whereas the harsh light of HPS or MH would actually burn the little babies.

I mix 1/2 regular potting soil with 1/2 seed starting soil ( from wallmart or lowes) as I find that seed starting soil by itself is just too light and will not drain very well.

Just my two cents woth