There could be 3 reasons why you cannot edit a post:
- an admin has edited it (added a category or similar also counts as edit). After an admin has edited a post the post is locked for further edits
- You also cannot edit a post if:
The maximum letters you want to change on trust level 1 is higher than 100, trust level 2 is higher than 400.
- You posted it more than 60 days ago.
This is so that people cannot rewrite their entire posts in discussions.