Well technically, no - but Buxus sempervirens wants to get 16 feet by 16 feet or more over time, so if you've kept them more or less the same size they were 15 years ago by keeping them rigorously trimmed as topiary, and they're in the same size pots, its possible the older wood in the middle is no longer productive, and that's why you're getting growth on the outside and not in the middle.

Thanks bamboo, if pruned back into old wood would they regrow

Yes, they do regenerate quite well - do it in winter though, maybe December or January. And they will look absolutely ghastly for a significant time afterwards. You may need to risk root pruning too though, always a dodgy exercise, I never get it right! But as they're in pots, it may be essential... some information in the link below, might be useful, you'll need to scroll down to the right bit...

http://www.boxtrees.com/topiary.html#Growing

but it doesn't tell you that you can actually cut these down to about 8 inches and they'll regrow, that's more for ones not topiaried at a small size and in the ground though.

My mate was fortunate enough to be passing a skip about 8 years ago. In the skip were about 15 box trees. He asked the owner if he could take them. It turns out they were a small hedge along the footpath leading up to the owners front door. no longer wanted... dug up... dumped.

My mate rescued about 8 of them. He trimmed the roots to a size to fit a small bucket... that is, 1 tree to 1 bucket. Finger pinched the growing tips.

Now 8 years later he has 3 very nice bonsai Box trees, 2 reasonable bonsai Box trees, and 2 err... ugly box bonsai trees (1died).

They are about 8"-10" tall, 10"- 14" canopy spread, and are estimated to be about 30 years old.

Nice freebies, huh! ^_^