[iBook] New RAM not recognised

[ibooklist at matthurst.fastmail.fm]

I bought a 512meg memory card from Crucial for my G4 1.2mhz iBook and
installed it exactly as per manual, as far as I can tell. Several times,
as it turned out, as the iBook thinks the slot is still empty - it just
shows the 256meg built in RAM.

Any tricks or tips? Or is it just knackered chips?

Matt