this has been a problem for awhile, there is a few quirks
first of all notice i marked it resolved
you can only mark it resolved it you navigate to the thread and use the link there, you can't mark it after you make a post, and you have to navigate to it fully, like click on coding, then click on particular thread, then use the resolved link
to make sure it should show the threadid in the address bar for it to work and also to say is_resolved is in there also
but i guess in short, just navigate to the thread and click the mark resolved link, but not after you post thanks or whatever