Warlords of Draenor hotfixes have been published the 08/25/2015 ! Here is the patch notes :
Patch 6.2 Hotfixes: August 25 (Source)
Raids and Dungeons
- Fixed an issue where Archimonde was not spawning on Raid Finder difficulty.
- Defeating Archimonde on Raid Finder should no longer prevent players from being eligible to obtain Crystallized Fel if they defeat Archimonde again later that week on Normal, Heroic, or Mythic difficulty.