Apple ID working for iPhone but not for iPad
One Apple ID can be used with multiple devices and if you are hitting a limit that too many devices are using it for a specific use (say playing purchased music on too many devices - no slots are left to authorize), you should get an error message.

The likely culprit is a network or other device specific error, but you may have to troubleshoot this:


http://support.apple.com/kb/HT4895


Most of the authorization issues can be solved starting with the above guide and general troubleshooting might be to check that you can browse the iTunes store and ask the networking staff to make sure things aren't being blocked or take the device to another public internet to be sure it's not a problem with the network and not the device.