assuming your talking about swapping a single 128mb stick with a 256mb one: its pretty normal,
not all chipsets back then supported higher capacity sticks, or even double sided sticks.
there were also a lot of "shady" manufacturers using double density chips, intended only for ECC server ram on non ecc sticks (which should never have had double density modules) - something down to the way desktop chipsets addressed the ram; often such sticks were sold with a compatibility list that mostly consisted of via and a few sis chipsets.
if you are lucky, you just see half the capacity you would expect, unlucky and it just dont work.
you can try increasing the chipset voltage and loosening timings, sometimes reducing the FSB would help also, if you have the options in bios (or, gasp, onboard jumpers). else, you might try digging up the boards manual and checking its compatibility list. you might well find that the board was only every designed with 128mb in mind!
best of luck
