Answer: The answer to this really depends on what kind of shrub it is. In general hedges tend to grow thin if they are planted in too much shade. This often happens over the years when nearby trees grow big and begin to an area that used to be sunny.
Privet, yew and to some extent arborvitae hedges can be pruned to encourage denser growth. These should be shaped so they are narrow at the top and wider at the base. This enables sun to reach the bottom of the shrub so it will stay dense.
If you can find out what kind of shrub you have and write in again using the Q&A form we can try to be more specific for you.
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