Hi there Ray,
hope you’re doing well!
Actually the problem was simpler 🙂
In my setup between the OS unit and the valves there’s a 25 meter run of an Ethernet Cat6 cable.
I used one of the wires as the common and the other 7 for the 7 sectors I need.
It turns out that after the 3-4 years some oxidation of the exposed end of the wire near the valve had caused a further drop in voltage.
Cleaned the wire and soldered it to the valve et voilà all is good.
Waiting for the same thing to happen to any of the other 6 sectors as I’m lazy (and really busy) 🙂