Craigslist always places the most recent ad at the top of the page. As the number of ads increases, your ad will move down on the list. If you want that your ad should be visible at the top of the craigslist, then you can remove your ad from that page and post it again. After the posting of your ad, You will receive an email from the craigslist which also contains the information on how to remove your ad so follow those instructions and remove your ad from the craigslist.