Whether you are installing DIMM or SIMM memory? Seems that your mainboard have an Intel 430VX or VIA Apollo VP chipset which cannot support 64 mbit SDRAM modules.
Also it can be a BIOS issue, check amount of installed memory in Windows.

Via Apollo VPX can be coerced to see 256MB in a single sdram slot if you use a dual sided 512MB module, at least that was the case with my Soyo SY-5EAS board.

The boards usually cant cache more than 64MB so unless you have a K6-2+ or a K6-3(+) CPU with L2 cache more memory than that wont do much good.

I found yet another mobo and it seems fine there.. must be something specific to these. I thought they were dead originally and they booted so it must just be a ghost in the machine..

Most likely the modules are incompatible with the mainboard/chipsets to a certain extent. In that case it is quite common that modules are detected with lower than nominal capacity.
You should post detailed info about the mobos and memory modules if you want to have a specific answer.