Motherboard only recognizes 128 of my new 512MB SDRAM

Hello,

I went through related messages in this group, but couldn't find the
answer yet. Any help is very welcome!

Here's my problem:
I have a VIA® Apollo Pro133 chipset (mobo model 693-686A), with
following characteristics mentioned in the manual:
• A total of three 168-pin DIMM sockets (3.3V SDRAM types).
• Memory size up to 768M Byte.
• Supports SDRAM at 66/100/133MHz.
• Supports Symmetrical and Asymmetrical DRAM addressing.
• Banks of different DRAM types and depths can be mixed.

Up until now, there was 2x32MB of RAM. I bought a 512MB SDRAM stick
and added it to my system. Unfortunately, it only recognized 128 of
the 512MB (so a total of 192MB; this is what the BIOS startup tells
me). Even if I removed the old sticks and put only the new one in
socket #0, I had 128MB of memory. I could accept the machine
recognizing 256 (because it supports up to 768MB in 3 sockets), but
128MB seems very weird.

This is (part of) what the CPU-Z program (system analyzer) tells me
about my system:
Module 0 SDRAM PC133 - 512 MBytes
Module 1 Winbond Electronic SDRAM PC100 - 32 MBytes
Module 2 Winbond Electronic SDRAM PC100 - 32 MBytes

Where do things go wrong?