I had a dozen Ilex meserveae 'China Boy' and 'China Girl' planted about 6 years ago. They are planted along the east and north sides of my house as part of the foundation plantings. Two of them look wonderful and have grown large. Two died. The rest are very unsightly and shaggy,. They have mostly bare stems with clusters of leaves at the ends. What's strange is that they are all growing in the same area with the same soil, fertilizer, and water. They all get about the same amount of sun each day. Also growing in the area are azaleas (some look great, some died, and some are so-so), fothergilla, vibernum, Magnolia virginiana, stoke's aster, pulmonaria, hosta, heuchera, lily of the valley, pumula spruce, Japanese Maple, spriea (lemon princess, little princess, and Anthony Waters), and lots of daylilies.
Can I cut the holly back to encourage it to branch out and form a more dense, leafy shrub or should I just remove them? Any ideas why this happened?