Hi Carlo,
1. The devices need to be added & licensed on Aruba Central.
2. The IAP's need to have a DNS server configured on them so that they can resolve the URL's needed for communicating with Aruba Central.
3. The clock should be correct on the IAP's to ensure the SSL handshake does not fail.
4. If possible, please upgrade the IAP's locally to latest firmware as well.
5. In case, there is a firewall on the network, it needs to allow traffic from the IAP's to internet.
http://help.central.arubanetworks.com/2.3.4/documentation/online_help/content/getting%20started/custaccounts.htm