Glad you care about your rhododendrons! Rhododendrons are not heavy feeders so the lack of blooms on some of your plants is more likely from lack of sun, water or bud damage from cold weather. Some varieties (mostly red and purple bloomers with dark leaves) need more sun, up to six hours than the shade tolerant rhododendrons such as the pink blooming Christmas Cheer rhododendron. Don't let the roots dry out in August or September when the plants are thinking about flower production. The best time to fertilize is early spring before the buds swell using a slow release fertilizer made for acid loving plants. Keep growing, Marianne Binetti