Hopefully you don't prune it at all, as this would remove flowering wood. Was it in flower when you bought it? Or has it never flowered? Is it in enough light, as although these are good in shade they do need a certain level of light in order to flower.

Hi Susancarol and welcome to GoY is your soil acidic or alkaline. If it is alkaline the rhodo will be struggling and wont have the energy to flower. Acidic soil, dappled shade and don't allow to dry out. Or is it in a pot?