What kind of soil do you have? Are rhododendrons / azaleas generally growing well in your neighbourhood?

I'ts rich humus ground and in the rest of my garden they are doing very well. Maybe the problem is caused by the fact there are large trees around this rhodo's?

Competition from tree roots for water and nutrients can certainly make life difficult for rhododendrons. The photo, however, shows what looks like a pretty healthy plant - good dark green foliage, lots of new growth. The somewhat sparse appearance is most likely the result of reaching for the light. The long willowy new growth in the lower right of the photo is a good example of growth stretching out toward more sun exposure. Limbing up the tree(s) casting the shade might help, but this sort of growth pattern is quite usual for rhododendrons grown in shade. Pinching the tips of new growth in the spring can also produce a denser, bushy plant. Sometimes it's also possible to direct growth more upward by using discreet stakes and ties.