It's probably too much heat, you wouldn't believe how hot the green house can get. I have a thermometer in mine and it gets up to around 119 degrees in there. That's too hot for small trees and cuttings. It can also be that the humidity is too low, you can try putting a plastic bag over the cuttings to make like a tent so that the humidity rises and slowly over a week or two raise the bag up so the tree gets used to less and less humidity. Good luck with them.