If you have a provider that appears in the provider list of the H&P section of a patient chart but not as an option for the main doctor in the demographics section, then you need to make them a supervisor.
To do this, as an admin level user, go to the manage providers option of the admin menu. Select the provider's name and click edit provider settings. Under the Title option there is a box next to the word supervisor. Check that box and click update provider info.
Now go back to a patient's demographic section. The provider should appear as an option under the list of main doctors.