I’ve had the same problem with my own plugin. Last summer when I uploaded v1.1.3, the “Last Updated” date correctly updated fine.
Earlier this month when I uploaded v1.1.4, the Last Updated date remained/s stuck at last summer’s date. The updated plugin does indeed show/list properly as 1.1.4, and downloads properly too. The pie graph showing the breakdown of versions also reflects correctly.
Did I update readme.txt? Sort of. I update that separately from plugin version upgrades, since I just need to declare that the latest version of WP works with my plugin (“Tested up to” line).
I wonder if the cause is this… I use the “Trunk Method” whereby I always have the latest/current plugin version in the trunk folder. In other words I don’t retain and tag past releases. In readme.txt I have “Stable tag: trunk”. Maybe the Last Updated field looks for a change in that line?
– WatchCount.com
Edit. I just found this post. A moderator says it’s a known issue happening to many other people as well. It’s in queue.