[FS1 Neural light not working]

Hi I am hoping for some help. I cannot get my neutral light to come on. I've tried  two new switches in and it doesn't work. However if I take the lead on the top and attach an earth  it does. I can see from the wiring diagram that the switch just makes an earth connection. I seems like the "probe" on the switch isn't meeting with something in the gearbox and closing to make the connection to earth. Any help very much appreciated