Feedburner checks daily for new updates. The time it checks depends on what you set it to in your settings. In your Feedburner setting you can tell it to check between a certain 2 hour time frame. So for one of my websites it checks between 5am to 7am eastern standard time.
That means if I want content I just publised to go out to people who are subscribed I need to publish before 5 am. If I publish something at 11 am it won't be sent out till the next day.
Check the time Feedburner is set to and change it to whatever you want.
Publish before that time if you want to content to go out same day.