Are you not able to put them under flouros? Clones don't really need heat from above. At least not like the sun generates. Flouros don't get as hot. In some places, geographically speaking, people put clones on top of heating pads because roots will go towards the heat. I don't do that because of my warm climate but I have about 90% success...maybe higher...on my clones. Clones need 24 hr light initially.
Another question would be... Are you keeping them moist? Clones like to be moist...not soggy, moist.
How old are your clones?