As above, RCDs work on sensing a current imbalance, so if there's no current flowing there's no imbalance.
Short live to earth, and a large current flows through the live, but none through the neutral, so the RCD trips.
Short neutral to earth, and, as both are at the same potential, no current flows, so the RCD doesn't trip.
If there is a load on the circuit, there are equal currents in live and neutral conductors, so the RCD doesnt trip.
Short neutral to earth on a circuit carrying a load, and part of the neutral current flows through earth conductor instead of neutral. The RCD sees an imbalance and trips.
So an RCD trips on almost any live fault, but only trips on neutral faults if the circuit is carrying a load.