Drooping yellow rhododendron leaves.

Is it in acid aka ericaceous soil?
THis is the most likely cause. YOu can buy sequested iron from garden centres which you can water on to make the soil more acidic.
If its in a pot then it maybe worth removing, knocking some of the normal soil off and replanting it in ericaceous soil.
If its in the garden and the soil is neutral to alkaline then sequested irons the way to go. A pH test would tell you if its acid or alkaline

you will probably find this is a feed problem, especially if you are watering with tap water they need feeding regularly.

First, most plants look sad and a little tatty after they've flowered. Second, rhododrendron likes cool, dampish, woodland conditions - I don't know where you live, but in the south it's been unbearably hot and dry.
If you've not got any yet, go and get some feed for acid loving plants ( for ericaceous plants like camellia and rhododendron and it may actually say ericaceous on the container) and apply that, and water if necessary (now we've had plenty of rain, though still not enough, really)