Teaching in multiple HM-Sec-SD-2 into one group?

[bosschaert]

I have multiple 3-packs of HM-Sec-SD-2 smoke detectors and would like to teach all 9 of my smoke detectors into a single group so that they all go off when one does. I read the manual and was able to teach 3 of them into a single group.
However when I started on the 4th, this one didn't seem to want to join the group of the other three.

Basically this is what I did:
1. I reset the 4th smoke detector by pressing the button until until it flashes orange and keeping it pressed until it flashes red
2. I press and hold the button of an existing smoke detector until it flashes orange.
3. Then I press and hold the button of the new (4th) smoke detector until it flashes orange.

However what happens now is that the light on the existing smoke detector immediately turns red (where it should turn green) once the light on the 4th smoke detector starts flashing orange.

Am I doing something wrong? Does anyone know how I can add my 4th (and the other) smoke detectors to this group?

Many thanks!

David

PS I can read responses in German too.