If each consumer unit has an RCD, or the circuits use RCBOs, they are probably blocking the signal.
If the house is wired with split load CUs, typical for 16th Edition wiring, where only the socket circuits have RCDs, then powering the units from the lighting circuits may mean there are no intervening RCDs stopping the signal passing between units.
And sometimes these units just don't like ring circuits (or some ring circuits) but radial power circuits are okay.
