Isn't it becasue there is a setting that limits the number of times your sig will show up in a thread? Ussually its the first and then after that, it does not show up. It might be related to how often in the thread you post. So if you do one right after another only the first will have a sig. If you come back tomorrow and post you probably will get a sig as enough time has passed to allow the theoretical page count to increment and so your sig becomes valuable information once more. At least thats how I would set it up if I ran a board.
Aaron.