Ensure that all files listed in the assets array are available to download. If any of these files return 404 or such (including the icons) the entire install will fail. You must either (A) make those files available or (B) delete those missing entries from the plist. The icon entries are not required for the download to work.
http://stackoverflow.com/questions/13167915/ios-enterprise-ota-distribution-unable-to-download-application