In a Drupal 6 site, I enabled the comment settings for two content types (con 1 & con 2). After creating a node in con 1 the "Add comment" button is showing at the bottom, but in the case of con 2, after creating the node, it shows a comment form instead of an "Add Comment" button. Both of the content types have the same settings as follows:
Default comment setting: Read/write.
Default display mode: Threaded list - expanded
Default display order : Date - newest first
Default comments per page: 50
Comment controls: Do not display
Anonymous commenting: Anonymous posters may not enter their contact information
Comment subject field: Enabled
Preview comment: Required
Location of comment submission form: Display on separate page