Sounds like they got the rot, very likely an embolism caused it...
Was the soil sterile?
Just for ref, and embolism is when you make the final cut but you get air in the stem... it blocks the phloem and xylem... makes a place for fungus to attack... eventually the cutting will rot below the soil line.
Try using clonex or another gel instead, because the gels help protect your cuttings from this.
I very highly recommend this method:
http://boards.cannabis.com/advanced-...ng-method.html
It's the one I use and swear by. I've been getting 100% rooting and survival since I switched to it.