Thanks for writing. This is a common situation that you can run into with any type of hedge. Constantly pruning the edges of the plant shades out the middle, and prevents new growth inside the plant.
To keep this from being a problem, try to prune your plants more evenly. Instead of taking all the growth from the tips of the stems, for example, cut some back farther inside the plant so you have an even canopy. It'll make your shrubs look thin the first year or so, but once light penetrates the center of the plant, you should get new growth in there again.
---Justin, Senior Garden Editor, BHG.com