The reason that they aren't showing up is because they may not be listed in the correct School Level/Department. If you navigate to:
  1. Core > Users > Edit User Profile Data
  2. Search for the Faculty Member and Select their name
  3. Scroll down to System Information, Click Employment
  4. Check to make sure the Department Selected is the same as the one in the Directory Widget
  5. Mark them a member of the correct School Level Department Effective immediately, if they are not already marked
  6. Click Save & Exit

If you would prefer not to add teachers to departments outside of their selected School Levels, then navigate to:
  1. School Website > Website > (Site Name)
  2. Navigate to your Faculty/Staff Directory
  3. Click the Wrench and Select Edit page Content
  4. Click the Gear Wheel for the Directory
  5. Mark the School Levels to include
  6. Click Save
This will bring missing staff members into the Directory.