It might be a browser caching problem or an OSMF issue - I tried to replicate it by clicking on the same 2 thumbnails for a few times in a row in FF and it seems to continue downloading.
Please raise an issue report at http://bugs.adobe.com/jira/browse/ST , so we'll be able to better track and investigate this problem.
Thanks in advance,
the reason why it works now is because I uploaded a modified version. I append "?<random number> to the URL. That prevents caching problems because the browser hasn't got the version cached. But I still report it as a bug.
I undestand that, it's the same solution I use if the browser cache is enabled and I want to ensure a refresh.
Thank you for raising the bug, It will allow us allocate time to investigate it.