Every motherboard has a specific amount of RAM it can support. If you install more than that it will fail to boot, report an error message, or only display the maximum amount supported as usable. Without the model number it's impossible to say in this case, but it sounds like your old computer cannot handle more than 256MB, which is about right for a model that old.
i have a pentium 2 dell computer with a processor speed of 233MHz
but when i want to upgrade my ram from 256MB to 512MB the ram does not change. if tryed on onother computer the ram is able to upgrade,
i tried all my techniques but its not working.how do i do this opgrade?