iBook memory upgrade

I've been trying to upgrade the memory on my girlfriend's iBook 700mhz, but I'm having trouble. From the research I've done, I found that the laptop has 128Mb of built-in SDRAM, and the extra slot can support a 512MB chip of PC-133 SDRAM.

Right now, the computer has the 128 built-in and an extra 128Mb chip (and the system profiler reads the total 256Mb,) when I put in the 512 chip, the profiler only reads the 128 built-in and not the extra 512.

Is it that the new 512 chip is faulty, or is there something I'm missing?