Is Caching enabled?
WebPlayer users may disable Caching for all WebPlayer applications by right-clicking on the WebPlayer content, selecting "Setup...", and checking the 'Disable all caching' checkbox. If caching is disabled, WWW.LoadFromCacheOrDownload() will continue to function, but will never cache downloaded AssetBundles. Caching is always enabled for PC/Mac Standalone and iOS/Android player builds.
if (!Caching.enabled)
GUILayout.Label ("Please enable Caching for the best gameplay experience.");