What are the possible reasons that Aruba Switches (2920, 2930, 2540) doesn't come up in the Aruba Central?
Aruba 2920, Aruba 2930, Aruba 2540 switches should have WB.16.02.0012 software version or later.
Switch should have access to NTP server in pool.ntp.org.
Clock should set correctly in the Switch.
The Switch should be properly provisioned with Activate.
Aruba Switch will not come up in the Aruba-Central if it doesn't have above criteria.
Below commands will be helpful to get additional details from the Switch.
debug destination session