DIAGNOSIS The boundary signal is too weak where the mower is operating. This is often caused by a wire that is too deep or a wire break that hasn't fully severed the circuit.
FIX Check the wire burial depth. It must be 1-3 cm (approx 0.5-1 inch). If deeper, the signal attenuates. Re-bury shallowly along the fault area. Ensure the wire is not running parallel to and too close to buried metal like pipes or rebar.
IF THAT DIDN'T WORK A partial wire break can cause a weak signal. Follow the wire path and check for damage. A break locator will find this.
HEADS UP Increasing the boundary signal strength in the settings is a temporary fix for a physical problem. Find and fix the weak point in the loop.