The new ram might be clocked higher than the old. So say you have the new one clocked at 133, and the old at 100 .. the mobo might switch to the highest common denominator thus disabling the slower RAM.

You weren't completely explicit how much ram and over how many chips you had in their originally so it is really hard to give a complete answer.

Note: DO NOT REFORMAT, THAT GUY IS A MORON.