Why does one of my feeds only display older items when I know there are newer items available?
Some feeds publish items out of order. For these feeds, enabling the 'Full fetch' feed option resolves this problem.
You may also need to set the item sort order feed option to date (newest first).
Try running the feed diagnostics in the context menu for a feed. It will display a message if this type of feed is detected.

Comments
Make a global option to sort by newest?
Rather than having to go through each feed and into options to set order by newest, why not offer the option to make it a global option? Or just ingrain it by default in the app?