Ease-up on lighting. The idea is to get the buried stem to produce roots, and keep the foliar growth to a minimum. I veg with a 400w MH, which remains about 12-18 inches from my rooted youngsters. Unrooted clones I keep at around 24 to 30 inches. Just enough light to keep 'em alive, but not enough to get mad growth above ground.

Keep the enviornment humid by using a dome or cover, but don't spray the clones every 20 minutes. Humidity stress (constantly wet leaves) will prevent the leaves from 'breathing', and it will eventually stop taking any moisture from the soil. This can stop all root development, and cause mold or fungus to grow. Remove cover for a few minutes, once a day at least, to provide fresh air.

Also, I scrape the lower part of the stem, down to the cambium layer. (just below the stem outer layer) I lightly scrape the bottom 1 inch or so of the stem with scissors or a knife....on opposing sides of the stem. Don't go overboard. Just enough to remove top layer of stem. that will get coated in the dipping gel, or whatever you use. It is this layer that produces the root cells. Stem layers - Cambium Also, keep in mind the pix in the link are a tad out of scale for cannabis clones, but it gives you a good idea what I'm talking about.

Like cigarettes42 mentioned...keeping the air bubbles out of the stem is rather important. Vegetable matter, along with stagnant air and water, usually results in rot. If you purchased the clones, I personally would still make a fresh cut while the stem is underwater, and let it remain in the water till it's time to dip and plant 'em.