AP15 inactive, deleted and never appears pending access points list

Hello,

I'm on Sophos UTM 9.406-3 with this problem:

3 AP15 become inactive, so I have tried to deleted each one to add again.
The problem is that each AP15 disappears and never return availave in pendig assess point list (under Wireless Protection).
I tried to restart each AP (turn off and, wait few minute and turn on) but with no success.
The only LED on the AP blink green and amber.

Please consider that another AP50 on the same LAN work without problem.
Any idea? I read about a Sophos tools to reflash firmware but AP15 isn't supported.

Maybe i need to return these AP to Sophos? How to?

Thanks

Filippo