If it's an old G, like the DW5200c or 5600c (screw back) the piezo-electric element
is in front of the movement, and not on the caseback.
If you have a back that's mounted with 4 screws, than Steve is spot-on,
and is probably is the little spring missing or hiding under the ruber bumper.
Anyway, a shot of the back can help.
G