I have two comment types on a node, one is review and the other is discussion.
When I manage display for the comment types, I selected Full content, teaser and token to show so that I can show different displays throughout the site for the comments.
I set this up and I lost the username and the original display for comments.
This is a different query though, but anyway.
I then shown full content for the node type the comments were on, all was great. You can navigate to the content page and see the comment forms at the bottom when logged in and the comments already made.
I then removed the full content, teaser and token displays as I wanted (disabled) as I wanted to see the original comment display again as it had more info on it, username, date, etc, which the full content display couldn't seem to replicate.
So I disabled them, went to the node with the commetns at the bottom and instead of seeing the comment forms and the original comments I had already made for testing, all I see is the link "add new comment".
I have now removed the Links field from the content manage display and i'm seeing all the comment full display again forms rather than just a link to "log in or register to post comments".....