When you create a discussion topic you have the option of adding participant poll. If you have added a poll to a topic after the discussion topic has been posted the board the poll will not show in the topic.
A poll is created on the board by copying the poll from the topic when the board started. This method ensures you can reuse the plan topic without affecting the current board poll results.
The effect of this is that you can add a poll to a plan topic before it is attached to a live board, but not once the plan has been attached to a live board.