Hello Roger,

You probably nailed it that there is a problem with the roots -- especially if you see a circle of cracks around the tree base about the size of the original pot that the tree came in.  This can happen when the roots have already started circling the outer boundary of the pot it came in. They can continue to circle even after planting. Or, as you considered, that hard pan soil beyond the hole dug for the tree is not inviting the roots to expand.

Since it's healthy right now, it may be worth attempting to correct the problem. And because it's so big, I would suggest getting the help of a certified arborist who has the equipment to dig out the tree and evaluate its roots. They can propose and execute a solution. Here is a link to information about how to find and hire a professional.

I'd do it sooner rather than later because a good stiff wind can blow large trees like yours over. It's a hazard in its current condition now.

Good-luck!