Rhododendrons and azaleas set flower buds in the late summer/early fall of the year prior to blooming. The number one reason for lack of bloom is inadequate sun. This is more true for azalea types than broad leaved rhododendrons. The second reason is too much nitrogen which encourages vegetative growth at the expense of flowers. Bone meal lacks phosphorous and is not as good as more balanced fertilizers such as HollyTone. Using any fertilizer without the guidance of a soil test is not a good idea.