1) First check that the battery is OK and that it has been wired correctly into the circuit.

2) Check that the thin wires to the coils have had their insulation removed correctly from the ends connected to the circuit. Carefully sandpaper the wires and solder them in place. This is a little fiddly and is a common electrical problem. The wires to the earpiece are very fragile as well, so it's a good idea to also check these.

3) Make sure that the earpiece is a crystal earpiece - normal personal stereo and radio earpieces won't work well in this circuit.