It means the spring-loaded ball inside the mechanism has failed. This ball is pushed into a indentation on the axle, which produces the slight click noise and feel in neutral position. You might be able to poke and prod it back into position, but once failed it'll pop out again soon.
The units are the worst example of service-hostile design (the gearwheels are crimped onto the axles) and can only be replaced as whole at an abhorrent price, unless you fancy some pretty major reworking of them.
