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Your shrub has sooty mold fungus. Neem oil can help. Here is more information: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm

Hopefully these two articles will help: https://www.gardeningknowhow.com/ornamental/shrubs/holly/growing-holly-bushes.htm

https://www.gardeningknowhow.com/ornamental/shrubs/holly/diseases-of-holly-bushes-pests-and-diseases-damaging-holly-bushes.htm

You may have mealybugs...This article talks about mealies on houseplants, but it's the same for landscape plants: https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

When hollies are thin, it's a good idea to give them a shear...just a light all-over shear. This encourages the shrub to branch and thicken up. The best time to do this is in early spring before first flush of new growth. However, it is not too late to do it now. I wouldn't wait any longer. If done now, new growth will have time to harden off before cold weather. If you wait, tender, new growth can be damaged easily by an early frost. I suggest that this be done around the middle of March, that's when our 6b zone flush starts. This article has holly pruning information:https://www.gardeningknowhow.com/ornamental/shrubs/holly/trimming-holly-bushes-how-to-prune-holly-bushes.htm

I tell customers that it takes 3 years for a tree or shrub to become established enough to grow at it's normal pace. If you give your plant a 'gentle' shaping mid march, you should see new growth that fills in to make the plant thicker.

It may be from holly midge. Here is more information: https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-berry-midge-pests.htm

Hummmm, that sounds strange. Is the entire shrub affected, or is most of the damage on one side? If the damage is on one side, my first thought would be that damage from herbicide drift has occurred. The main reason I think this is from your statement that the new growth looks good. Glyphosphate herbicides, ( like Roundup ) only kill foliage, not roots and woody stems. Also, why would shrubs in two different areas exhibit the same problems? Obviously, it isn't location. If the shrubs were mine, I'd wait to see what happens. If the new growth continues to do well, you can trim mid-March to eliminate bare stems. If the new growth starts spotting and turning brown, I'd take samples to your local agricultural extension office for identification and recommendations.