@GoneButNotForgotten I do not recommend root pruning. What Gardenman said; but if really concerned, I would examine the roots by digging it up & making sure all roots are pointed away from the trunk. If it was a containerized tree originally, it might have had the roots encircle the trunk which could impede growth by causing girdling.
A balled & burlapped tree would have had it's roots cut originally when dug out of the ground.
Examine the root system & see if that could be the problem, especially if you are treating them the same.
☼The best place to seek God is in a garden. You can dig for him there. GBShaw☼