Not enough humidity and hydration is the short answer. Tom has written a lot on the subject and has threads where he has done some experiments on his theory and proven it to be correct. Just check out his profile, look up his threads and you will find them. Read up, there is lots there and very informative. Happy reading!
A long time member who I pay homage to is "Tom"
He has great success using his hydration, humidity, warmth method in a closed enclosure and I did a 180 in my thinking about it.
I am getting 3 baby Sri Lankan Star hatchlings and am planning to do this method for the first year or 2.
Here is an Idea I have for a new closed enclosure to achieve the method. It's 4'x4' by 5' high
In the link below there is a cool video about this product. It will have to be modified but I was thinking the heat source could be underneath and the lights hanging inside from the top.