Trimming hedges to often results in a weaker bush with a thin green shell and hollow center with no inner leaves. If You can pull back some branches and see no inner leaves you have a problem. To fix this, and get better growth inside the center, learn how to thin out the right branches. This will cause the bush to make new branches inside. Its a good habit to start if you want to give some vigor to your bushes. Otherwise they are on their way out and will have to be replaced soon.