It is hard to tell from the picture. It just looks like it has a general lack of vigor. Here is what a rhododendron needs:

I. Soil Drainage: Because the fine roots of azaleas and rhododendrons are easily blocked by fungi, excellent drainage is important. To test drainage, dig a hole 6 inches deep in the bed and fill it with water. If the water has not drained from the hole in four hours, install drainage tile to carry away excess water, or build raised beds. Always plant the rhododendron higher that it was originally. Never deeper. Moist well-drained soil is a must for most varieties. This sounds difficult, but it means to not let the soil dry out completely but don't get it too wet. Thoroughly water if necessary and then let it become almost dry. Most gardeners do this by planting in a well-drained area and mulching to hold the soil moisture in. Watering is seldom necessary except during long dry periods.

II. Acid Soil: Most varieties require an acidic soil (pH 4.5-6). Powdered sulfur is the best agent to acidify the soil. Holly-tone has this in it. Your plants will get chlorotic if the soil is not acidic enough. That doesn't appear to be the problem with yours.

III. Fertilizing: When rhododendrons and azaleas are not planted in ideal locations they may develop chlorosis. Chlorosis is yellowing of a leaf between dark green veins. It is caused by malnutrition that can be caused by a wide variety of conditions. They include alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency, nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer. In any case, a combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Holly-tone contains these elements and 4-6-4 fertilizer. It is best applied in the spring prior to blooming to make sure the plant is healthy when forming next year's flower buds. If you missed applying it in the early spring, it can be applied up until mid summer. Rhododendrons do best when left alone in the right conditions. You don't need to use Holly-tone or any fertilizer unless the plant shows signs of malnutrition.

IV. Shade: Some shade; some varieties like full sun to bloom but others suffer from too much sun. This is a trial and error thing unless you know the variety and can look it up. More sun stimulates flowering and but may trigger lace bug infestations. Prune off lower branches of shade trees so that you have "high shade" above your rhododendrons. This is ideal for a healthy rhododendron bed.

V. Mulching: Rhododendrons do best when they have about a 2" to 3" layer of mulch to hold in moisture, prevent weeds, and keep the roots cool. Since most mulches are organic, they need to be topped off periodically, usually about every year or two. Do not make the mulch over 3" thick. Keep the mulch about 2" to 3" back from the trunk/stem of the plants to avoid bark split and rodent damage. Do not use peat moss as a mulch. It is a soil amendment to be used when preparing the soil in a bed and can cause severe problems when used as a mulch including dehydrating the soil and preventing moisture from reaching the soil. It also tends to blow around. It is best to mulch with a 2-inch layer of an airy organic material such as wood chips, ground bark, pine needles, pine bark or rotted oak leaves. A year-round mulch will also provide natural nutrients and will help keep the soil cool and moist.

VI. Protection: In choosing a location to plant rhododendrons and azaleas, protection is very important. Protect from winter winds. This is especially true when the ground is frozen. Also protect from deer. I use deer netting in the winter and for plants that are susceptible to damage in the summer, such as deciduous azaleas, I have a couple beds protected by deer fencing. Some people have problems with rabbits or squirrels eating the buds.

Here is a link that might be useful: How to grow rhododendrons and azaleas.