RAM won't work on cpu

RAM won't work on cpu, why?

I just got a new RAM but it wouldn't work on my cpu, why does this happen?

I checked my cpu and got the right kind of RAM which is pc100 512MB SDRAM 168-pin DIMM Non-ECC but still won't work. btw the maximum size of RAM my cpu can take is 768MB and I already have 256MB of RAM equipped and I'm adding a new 512MB but it wouldn't work.

Solution:-Let's see now. You forgot to tell us what brand an model your motherboard is... You did not mention what CPU and speed you are running. You also forgot to tell us how many slots you have. If there were 4, then identical pairs must be installed.

But here is a guess.

Plug in the new RAM by itself and try it.

Move it to a different SLOT. and try again.

If you can take those new ones BACK, you might look up the possibilities at http://www.coastmemory.com