why is my rhododendron not flowering?

For the last 2 years, my rhododendron impeditum stopped flowering; it is planted in a part, sun part shade location. There are 2 others in that garden, one is 6 yards away and another 30 yards from it.  There are many bees roaming the garden. The other two rhodo only had 2 or 3 flowers only this year as well. I put compost at the base of each in the fall, would that be the reason they stopped flowering? Any ideas?