At this concentrations, it should not be too much of a problem to measure Pb, even though GC electrodes are not very sensitive. Is -400mV a low enough potential to accumulate Pb onto the electrode ? Have you tried at -0.8V?
Also, I don't know if there is Cu in your solution but I remember having some disappearance of the Pb signal with addition of Cu.
If it doesn't work, maybe try a gold electrode which is a bit more sensitive to Pb. Or maybe modify your GC electrode by depositing some bismuth on the surface.