It seems, my good sir, you may be having problems with the log-on system: I'll see your activity labeled as "Replying to [insert thread here]" one minute, and you'll be logged out the next.

I've actually overcome this problem: when it says that you don't have the permissions to post, and brings you to the log-in page, you simply log back on: It posts your reply for you (well, for me, at least).