Why would you root prune? That may not be the issue. My large White one is about 4 now and I haven't once thought to root prune! Try upping the fertilizer if you're not feeding it often.

Spring is a good time to transplant them too BTW.

Because it's one big root ball? Little soil left. It's already in a 2' X 2' X 2' container. I don't have space to put it in a bigger one and I've been happy by the size it grows. I know they can get much bigger, but my yard cannot. lol