If you are seeing new vertical growth very early in cloning, the plant has too much nitrogen left in it and your light intensity should be decreased as well.

You may have rooting probs if the plant has decided that it wants to grow leaves, not roots.

Watch temps... with the summer coming on, I just lost a run of clones when my dining room hit 90'F while I was at work... CRAP. This method seems to work best of all when ambient room temps are in the upper 60s and the heat mat warms the inside of the dome just right, not above ~80'F.

I just went and picked up a quart of wilt pruf concentrate and plan to do a dip on the next run, with a couple drops per gallon bleach added to slow down fungal growth.

I will keep you posted.