well, I don't have one either, but I just put a pastic sandwich bag over the top of them as I start all my plants off in 16oz cups, and just spray some water in the bag and put it over top of the plant and use an elastic band to keep the bottom of the bag closed so the humidity don't escape and take it off for a couple mins. a day to let her get some fresh air and breath and put it back on, just seems like from my experiance this is the best and cheapest way for me to clone. I do the for 2 weeks and by then if they don't have roots, figure they're not going to so I toss them.
To answer your question, I think it's cause you don't have enough humidity, they love it around 80%