Rhododendrons are always fertilized after flowering, unlike other Flowering Shrubs. You should use a special Rhododendron fertilizer. Compost is not suitable due to its high pH. An alternative is a natural fertilizer such as GreatOrganics, where there is a wide variety of microbes that would also be delivered via compost, but without the negative side effects such as the aforementioned pH.
Garden owners can achieve the ultimate blooming by taking out withered plant parts regularly. This causes the shrubs to not waste energy for seed production and put all the power in the formation of new flower buds.