Poorly Rhododendron

Hi all - newbie gardener here, so apologies if this is a silly question!

Does anyone have advice about why my rhododendron has turned all droopy with yellow leaves as in the picture below? I've read a few articles that give some reasons, such as poor drainage, lack of water etc, but I don't think it's those as the surrounding plants (including other rhodes) are fine and I water it regularly (but not too often). The only explanation I read which it might be is that the root ball is planted too low, but what's got me confused is that there is plenty of new growth which looks healthy. I think my next move is to try and replant it a little higher, but just thought I would check here first in case anyone has any better ideas?

Thanks in advance!