Two pests that may cause what you are describing:
Junipers are a favorite victim of red spider mites. If the tree appears weak, with yellowing foliage, it may have spider mites. To check for spider mites, hold a sheet of white paper under a branch and gently shake the foliage. If the paper comes away with many small dots that move, it has spider mites. To combat spider mites, spray with insecticidal soap or a nicotine solution (which can be made by soaking tobacco in water overnight).
Juniper scale causes yellowed needles, and infected branches fail to produce new growth. The scale is round and at first white, later turning gray or black. Check for the tell-tale casings of scale insects on branches. Use a systemic insecticide.