Certified GKH Gardening Expert
It certainly could be rootbound. If it has spent more than a year in container without root pruning, then it may be time to do so, while giving it a fresh batch of compost, or nutrients.
1/4 cup of DOLOMITIC LIME and teaspoon of WETTABLE SULFUR will help to keep away any infections during the year. Use this treatment once, or twice per year.
This article will help you to care for a Holly: https://www.gardeningknowhow.com/ornamental/shrubs/holly/propagation-of-holly-shrubs-with-holly-seeds-or-cuttings.htm