Q.Thinning Azalea

I have several azaleas along the front of my house, all planted from pots at the same time about twelve years ago. A few years later we planted a Bradford pear tree in the front yard to provide shade on the house in the afternoon hours. Now the bushes get filtered sun until the sun drops low enough below the tree, which alows late afternoon sun to reach the bushes. Two years ago we had a severe drought here in NC and I was careful to make sure that they recieved enough water. The problem started before the drought though. At one point all the bushes were so thick that it was almost impossible to see through them, but in the last two to three years two of the bushes seem to have stopped growing and are extremely bare to the point that it is very easy too see through them. I have tried azalea food and basic fertilizer, like 10-10-10, to no avail. What gives? Any help would be appreciated.