Please verify that all Access Points that you have are:
- of the type Aruba Instant, which can be seen from the part number IAP-275-RW
- for the same regulatory domain, which is the last part of the part number: US=US, JP=Japan, IL=Israel, RW=Rest-of-World
- in case you have different types of AP, like 225 and 205, make sure they initially run the same firmware before you try to join them in a single cluster. For the upgrade make sure you only have one type of AP active at the same time.