I am not responsible for the forum technology, but I will try to answer your question. So far as I know, other people cannot write in threads you've started, since this forum is only answerable by instructors. Check to make sure you're signed in to your account in relation to replying to an open thread you've started. As for "resolved" threads, they cannot receive extra replies, since the initiator of the discussion, upon receiving an instructor's response, has deemed it satisfactory, thereby closing the thread.

Does this help?