There may be a delay of up to 2 hours between the time you allow an ad and the time that ad begins to compete in the auction for ads running on your site. Also, please keep in mind that simply allowing an ad doesn't necessarily mean it will show up on your site; ads you've allowed in the Creative review must still compete in and win the auction to appear on your pages.
Ads that you've previously disapproved may take up to 24 hours before appearing on your pages.