There could be several reasons why your rhododendrons don't look happy...

Are you seeing light yellow leaves, sometimes with brownish burned areas, which will be more apparent on the sunny side of the plant? The under leaf sides, protected from the sun, would still be deeper green in too much sun. If that's the case, you may want to give the plants more shade or move to a more protected site.

Sun scald, with its yellowing of leaves surfaces and often with brownish burned areas is more common in spring on new leaves and in winter on leaves with winter burn. When rhododendrons are planted in the sun, a good mulch is necessary to keep the roots from getting too hot. Heat damaged roots will result in wilting and malnutrition.

Moving a plant to an area with more shade may alleviate the problem. However shade has its problems also. Some varieties of rhododendron don't bloom as much in shade.

Other things can cause yellowing of leaves:

1) Yellowing of a leaf between dark green veins is called chlorosis and is usually caused by an iron deficiency.

2) Yellow mottling on the upper surface of leaves and black sooty mold and transparent insects on the bottom are symptoms of Azalea whitefly (Pealius spp.) and Rhododendron whitefly (Dialeurodes spp.).

3) Yellowing and dropping of leaves is normal toward the end of the second summer on the small-leaved lepidote rhododendrons.

4) Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.

5) Yellowing of leaf edges has been noted in gardens where sandy soil conditions or root competition with other plants caused insufficient soil moisture and nutrients.

6) Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of rhododendron lace bugs, Stephanitis rhododendri, and azalea lace bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves. This insect hatches early in spring as the new foliage begins to mature and its numbers may build to damaging levels with successive generations. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, plant your rhododendrons where there is some shade.

If you are not getting sun scald, then the problem could be a variety of things. I have a trouble shooting chart on my website below.

Here is a link that might be useful: Common Problems and Their Solutions.