Hi,
What is the version that you are using? Are the 207 APs of the same regional code (Example RW or US) like the 215 that you have?
Do you have NTP configured (in case you were using cluster-security)?
Does the 207 have any previous configuration? Did you try to factory reset it?
Did you try to check the logs using show log system command?
I would contact Aruba TAC Support to have a look if the issue persists..