"Post a comment" is only available until the first comment has been posted. After that, you have to click on the "x comments" link to make a comment.
At least that's how almost all of the existing themes work. Geeklog 1.3.7 introduced a couple of new variables for use in themes, so that you could make the "Post a comment" link always visible.
And, of course, you can also disable the ability to post a comment on a per-story basis.
bye, Dirk