The Subject Type is most likely not associated with that particular type of case.  This is easy to resolve.

Check to see if the subject type that is not appearing in the drop down is associated with that type of case you are editing.

Click on Settings > then Case Types under Case Customization

On the Case Type edit screen and locate the Subject Type and Additional Subject Type fields.  Add the types of Subjects that are associated with that type of case and they will be available to select from going forward.

For additional information, check out this video tutorial on Subject Types