Have you ever had the problem with “No more mirrors to try” when behind a caching proxy? Especially with rpmfusion? You tried yum clean all, and still you are not able to download the xml or sqlite db?
To fix this you have to activate the http-option Pragma: no-cache of yum. To do this add:
http_caching=none
into the yum.conf file. At least for me it now works all the time todo a yum update.