Maybe you've tried this already, but you can get rooting compound (I think that's what it's called). You treat your stock with it, and plants that would not normally root can be coaxed into rooting. I have no personal experience, but got curious since legal weed here in Colorado often depends on cloning plants. I wondered, how do you clone a plant?
Simple, it turns out. A sprig of willow when stuck in damp ground will grow roots, due to the presence of some hormone (if I remember correctly). So you can brew from willow a liquid containing that substance, use it to treat another plant, and the treated plant stock will root as a result. Commercial rooting products are simply synthetic compounds based on that willow product. Perhaps it works on some plants and not others--I don't know.
You might look into this and try it. A simple internet search will explain things probably better than I have.
