Why can I never load certain sites?

There are some sites that never load for me, they always time out. Mostly https, but not all https sites. World of Warcraft downloader works fine, but I can never download using the Aion Online downloader. Aion Online account management works fine, but Runes of Magic Account management doesn't. What could be responsible? How can I figure out whether they're being blocked by the network firewall (which I have no access to?)