Currently Google is transitioning Google Apps accounts to a new infrastructure. If your account was transitioned by you or by Google and you are experiencing problems with mobile syncing now then
- go to the dashboard as an administrator (if you only have a user account ask your admin to do so),
- click on “Organization & users”,
- go to the services tab and
- make sure that “Google Sync” is activated.
- Go to Settings > Mobile and check if “Enable Google Sync” is enabled (which it should be if you have used Google Sync already)