Hi guys,
Thanks for your interest but it was more easy than that. One AP was in standalone mode so converting to IAP solved the problem. There's nothing in the GUI that says an AP is in standalone mode, and it shows all the options as it was an Instant AP (auto-join mode, preferred master, cluster options, and so on), therefore only by looking at Maintenance > Convert option you realize the AP is in standalone mode
Regards,
Julian