Thanks for writing. It sounds as if your rhododendrons may be hungry for nutrients. If you've not fertilized before, try feeding them with a product formulated for acid-loving plants --- that may help.
If not, I recommend you do a soil test. It may be that your soil has too high a pH for rhododendrons or is lacking a specific nutrient your fertilizer may not supply enough of. The soil test should give you that answer.
---Justin, Senior Garden Editor, BHG.com