Unless you have a mist propagation unit the only rhododendrons you have any chance of rooting at the very small leaved varieties, such as Impeditim, Blue Title, Yellowhammer, etc. With these varieties the cutting has to be taken the moment the new shoot ceases to grow, but before it starts to `firm up` - too soon and it will simply collapse, too late and it will be too hard to root. If you take about 100 such cuttings then you might just get one or two to root.