@Plo 

Hello, you can try to reboot the main Deco. Did you set up a static IP address on the new clients?

What's your phone's model number and system version? What's the Deco app version? 

What's the model number of the client that not show up on the app? It is connecting to the Deco with a cable or wirelessly? Which Deco does it connect to? 

What's the Deco's hardware version and firmware version? Have you tried factory reset it and reinstall?

@Stella.Q many thanks for your quick help. 
phones: iphone 8 and iphone 8+
no static IP

devices connected on wifi

all clients do not show in the app as if there were no devices conencted. 

I have not tried to reboot the main deco but i have switched off and on the whole system a few times to try with bo success

i was trying not to have to reinstall everything...