Threads that haven't gotten a reply in a while are locked automatically. How long this takes depends on the board. This is to avoid gravedigging, and it keeps replies more relevant to the situation. In this case, all posts in unpinned threads this board are nearly a month old, so chances are the posts you wanted to make wouldn't be relevant anymore to the situations the posters of the threads were in when the threads were made.
...Does that make sense?