I ran into an issue this week where iOS devices weren't able to open the Google Drive application. The users were getting the error "Device Policy Alert: Looks like your device isn't updated with the latest security policy. Please try again later.", despite the "Google Apps Device Policy Payload" being installed.
Long story short, the resolution was that the Apple Push certificate for the Google Apps domain had expired. Updating that fixed the access issues. Instructions for doing that are available on Google's support site: