When you add a new source, rasa.io automatically goes to pull from the source into the upcoming articles page. After the initial pull, the dashboard will pull from your sources a couple times a day until the set cut off window that you set on the schedule page. That means even if a new article is published from one of your sources after the cut off window but before your send time, it will still not come into the upcoming articles page.
Once you send your newsletter out, the upcoming articles page will clear out and refresh. The dashboard will start pulling from your sources again leading up to your next send to make sure you have the most up to date articles and information in your upcoming newsletter.
If you want to request a manual pull from one of your sources rather than waiting for the dashboard to pull, you can use the 'refresh' option on the sources page. You can do this by clicking the three dots next to a source and clicking 'refresh.'