Do you know what condition caused it to lose its leaves because leaf loss other than the oldest lowest leaves in evergreen rhododendrons isn't normal....if that wasn't corrected the same thing may be causing it not to thrive now.

Yellow leaves with veins remaining more green is chlorosis and usually indicates soil that is not acidic enough. Rhododendrons have low nutritional requirements, but may be unable to utilize magnesium and iron present in soil if the PH is too high.

And, anything that might damage roots can also cause chlorosis if the roots remaining are unable to supply the top growth with needed nutrients. Compacted soil, drought, fertilizer burn, nematodes or weevil damage to roots.