There are many critical factors: time of year, maturity and age of the wood, size of the wood, time of day, care of the wood, wounding the wood before planting, hormone treatments, planting mix, planting light, humidity, drainage, hardening off after rooting, etc. Many, many things to learn to become a propagator. The easiest way is to send cuttings to Kathy Van Veen at Van Veen Nursery in Portland, Oregon.
http://www.VanVeenNursery.com/rhododendron_propogation.html

The hard way is to learn to propagate. A primer is available at:
http://rhodyman.net/rarhodyhopr.html