Certified GKH Gardening Expert
This could be tough. Can you include an image so that I can see the surrounded area? If they are all the same type, then It makes me wonder if there is something planted near the affected few that might be causing an issue.
I can better help you if you could upload a photo. In the meantime here is an article for their care: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm