Your tree may have stem girdling roots. One theory of the cause of stem girdling roots is that they develop as a result of trees being planted too deeply. When root systems are too deeply burred , less oxygen and water is available. The roots will grow up towards the surface of the soil and  encircle the trunk. If you remove the root that is girdling the trunk then the tree may recover. The first site explains about stem girdling roots.

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/girdling.aspx

The next site explain how to correct this problem.

https://hort.ifas.ufl.edu/woody/remove-girdling-roots.shtml

If you are uncomfortable doing this then you could hire a certified arborist.

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

https://www.treesaregood.org/treeowner/whyhireanarborist

Hi - but I don't see any roots / uneven soil at the base.  Would I if this was the problem?

Thank you - two more questions - what is the bet time of the year to girdle?  Also, are there companies that you can hire for this service?

Thx.