@Rochitas 

Take the Deco unit back to the same room of the main Deco and plug it un to see if it does go green. If so, you may need to put the problem Deco closer to either the other slave Deco or the main Deco.

If the Deco is in the same room as the main Deco, and it blinks blue, default it by holding the reset for 2 seconds. The LED should go yellow, then it will start pulsing blue. Add the Deco back in with the app.