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

1. go to the dashboard as an administrator (if you only have a user account ask your admin to do so),
2. click on “Organization & users”,
3. go to the services tab and
4. make sure that “Google Sync” is activated.
5. Go to Settings > Mobile and check if “Enable Google Sync” is enabled (which it should be if you have used Google Sync already)