A few years ago, I purchased and planted 3 holly bushes, 2 female and 1 male, I believe. The 2 females are growing great!!! Getting taller and filling out. The male bush, however, is still the about the same size as it was when I bought it. I can't figure it out.
I would check the roots. There maybe something attacking the roots of the plant that is keeping the plant stunted.
Just as a precaution, treat the plant with neem oil. It will kill pests and fungus, which is also a possibility.