Your Rhododendron looks like it has stippling damage caused by azalea lace bug. This page from Solve Pest Problems includes photos of leaves up close. The black frass specks on leaf undersides is one thing to look for. The page goes on to offer solutions such as spraying water with a strong stream, insecticide (follow label for any product) or replacing with a more resistant variety. Check out this page from METRO, too.
I don’t think we can say with certainty why one shrub is more infected more than another. Some types are more susceptible. A shrub under stress is also more prone to pest attacks. It will take a year or more for the root system of a transplanted shrub to re-establish. Perhaps, with short term treatment, your plant will recover and fight off ongoing insect pressure.
If you compare photos with your plant and this isn’t lace bug, please provide leaf closeups and we’ll try again.