Certified GKH Gardening Expert
There are alot of things that can go wrong when you are trying to root cuttings and since I don't know your process, I don't know where it could be tweaked. But, I can tell you where things normally go wrong...
Rooting in water
Cuttings often fail in water due to bacteria and fungus in the water. It is very important to change the water regularly, possibly even daily for plants a that are prone to rotting. A drop or two of bleach per cup of water can also help keep bacteria and fungus from getting in the water and rotting the cutting.
Rooting in soil
Often, the reason for failure with rooting in soil is either failure to keep the soil moist, keeping the soil too moist or failure to keep the humidity around the cutting high enough. For the soil too moist, make sure that the soil drains very well. Adding some perlite to the soil can help with this. For too dry, check the soil daily and water when the top of the soil is dry to the touch. For the humidity, make sure the cuttings are in an enclosed environment, either under a glass cover or under plastic. Just make sure the cover does not touch the cutting as this can encourage rotting.