Cutting at a really gradual angle, or splitting the stem with razor blade or a scalpel can help, some growers gently rough up the stem with a razor blade edge. I know someone who gets almost 100% by sticking them in the gravel of his aquaponic system. Certain strains also clone really easy, some are very difficult. How big are they, I have found smaller shoots close to the bottom to clone easier than big branches.
Love the stoner cat!!