There are a few reasons this may happen:
- the board you're trying to post in is restricted and only the original poster or staff can reply
- the board you're trying to post in is "read only" and only admins can post
- the board you're trying to post in requires approval before the post is made visible
- you have received a warning and posting privileges have been restricted
- you are not a member and don't have posting priveleges
- you aren't logged into the forums