Many hollies including yours are dioecious i.e. the plants are either male or female. Yours is obviously female & needs a male plant nearby for pollination & berry production. Surprised the garden centre didn't warn you!

You can get some berries on a female without a male but to get a lot you need a male. Looked up suitable males for you and found Blue Prince or Casanova.

Thank you Steragram and Darren, your advice has been very helpful!
Thanks Steragram for doing research for me too! I will have look at both of your options.

they flower in spring so you may get berries later in the year especially if there are male hollies in neighbouring gardens.
But if you prune it to keep its ball shape then you may well be cutting off any stems with flowers. No flowers means no berries.

Thank you Seaburngirl. I don’t think any neighbours have holly growing, unfortunately! I bought mine already pruned into a ball shape, haven’t pruned it myself yet. I did consider keeping it in this shape or growing it sort of cylindrical. However, I’ll take your advice, I’ll leave it well alone and keep watching.

If you look at some of Hywel's b;ogs you can see a lovely holly pruned into a ball shape but allowed to grow tall.

Thank you Steragram, I’ll have a look at Hywel’s blogs and have a browse! Sounds good advice.