Certified GKH Gardening Expert
These are notoriously slow growing. If it is producing plenty of berries, then it is signalling that it is healthy.
Unfortunately, these are doing exactly as they should be, so in order to get a taller shrub, you will have to get a variety that grows taller, faster.
This article will give you more information on the different Holly types: https://www.gardeningknowhow.com/ornamental/shrubs/holly/common-types-of-holly.htm