There is simply
no edit button ... so nothing happens

(all the other posts have an edit button, only missing in the first post of a thread)
everybody can edit their own posts, except if the post is the startingpost (the very first in the thread)
"can edit posts" and "can edit own posts" is set to yes for admin/mod
"can edit own post" is set for the members
//edit
I just stumbeled over a forum where it actually is possible to edit the very first post in a topic ...
so it seems it has something to do with the forum (channel) options ...
any idea?