When a new thread is started or someone post a reply to an exsisting thread that thread goes to the top of the page. Therefore, since the last post in your original thread, there have been either new threads or new posts in exsisting threads that "bumped" or placed them at the top, due to the fact the forum sorts the threads by date (newest threads/posts first) so yours got pushed down.
Also if you have not done so, you need to set your own options to view all topics "since the beginning" (or something like that).. I believe the forum defaults threads from the last 7 days to be viewed unlesss you change it.