Was updating one of my Wordpress plugin, and find it weird that the Wordpress Plugin Directory does not reflect the changes correctly although I've changed the "Stable tag" in readme.txt.
According to Wordpress Plugin "Task 3: "Tagging" a new version" guide, you'll need to edit the trunk/readme.txt beforehand.
That is not enough!
In the end, I found out that you will need to update BOTH:
Hope this helps!