If you mean Leylandi, I'm afraid they do tend to develop a bare stem at the base as they get older, it's the nature of the plant.

You can get them to stay fuller at the base by cutting them so they are narrower at the top than at the bottom, rather than just straight topping them, but that's a prevention. To the best of my knowledge, once they go bare, there is no cure.

Agreed. There are few things which will grow close to a leylandi but I'm told Eunymus emerald Gaiety or emerald Gold will cope in the dry shade and you could use this slow growing shrub to cover the bare patches at the bottom of the leylandi

http://www.flickr.com/photos/31559373@N00/tags/euonymus/

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