First, log in with "Remember Me" checked. It's an idiosyncrasy of this board, you will get logged off and (and sometimes back) on automatically.
Second, some sub-fora don't allow any editing. None of them after 30 minutes.
Third, if you do write a post or edit, and are disallowed, if given the option, use the immediately provided logon screen to preserve and submit your post.

Good luck.