Do the trunks or branches of the cedars have lesions and are the branch tips dying back?
This could be cankers caused by fungi. Cankers can result in stunted growth as they girdle the tree and kill branch tips. To treat this problem, remove and destroy any affected branches. If possible, remove and destroy the cankered area, as well as 2 inches of healthy bark that surrounds the area. If a cedar is heavily diseased, you likely will not be able to restore it. It should be destroyed.