Thanks for writing! Rhododendrons typically do well without any special care in your area. It might be that the plants are getting old and lose their vigor; this is not uncommon. (Unfortunately, not every plant is as long lived as a redwood!)
You can also try doing a soil test and seeing what the ground is like --- a soil test will tell you if there are nutrients the ground lacks or has too much of. Many soil tests also tell you what and how much of it to add to improve your soil.