I have a dist fib fx web b non displaced non union will my ortho be able to determine exactly why it hasn't healed?

Generally: Nonunions come in a couple varieties but in your case is likely from an atrophic nonunion. If you smoke or chew that is your primary reason. Otherwise, sometimes its the type of fracture, insufficient immobilization, etc. Knowing the exact etiology is not quite as critical as treating it from here appropriately. Its usually just a problem with biology.