Some reasons that an IAP cannot join a cluster can be:
- joining AP is a different AP type and the firmware version does not match the cluster version (but you checked the firmware version)
- joining AP is configured as standalone AP
- joining AP is configured with a management VLAN, so it tries to get an AP on a tagged VLAN
- joining AP is of a different regulatory domain (US/RW/JP/IL). APs with a different regulatory domain cannot be joined in a single cluster
- joining AP is FIPS version, the cluster is non-FIPS version (or vice-versa). You can recognize FIPS access points on the -F1 in the product code on the back.
- joining AP is of a different brand. Dell or Alcatel APs cannot join an Aruba cluster.
A hard reset (with the reset button pressed while powering on, check the AP documentation for the exact procedure) may solve some of the issues, but I assume you did that hard AP reset already.
Do you have access to the serial console and see what the logs there are telling? Does the AP get an IP? In the correct subnet range? What does the log on the virtual controlled tell you? Do you see attempts to join the cluster?
If you get stuck, you can post the console log of the joining AP and the 'show log' of the virtual controller; or if you prefer not to post it on a public forum contact Aruba Support.