take the ram out and clean the terminals with a soft rubber and wipe with a clean cloth.
You haven't said what the "new software" is called? - is it Sandra? I use "Dr Hardware" from http://www.drhardware.com - Its free to try so get a second opinion. With both the above programs it can be confusing because you have to select at the top of the screen the memory stick to query, the data output usually shows up in the centre of the screen - thus one can think its only found one stick - therefore you have to manually select. A word of warning, these type of programs are not 100% accurate and have trouble even reading some hardware. So get that second opinion. If I were you I would solve this issue first before buying more.
OK here we go. I have a machine that has 512MB of PC2100 RAM in it. I have gotten some messages form the computer that say that there was a problem with the RAM. I didn't do a test, (very complicated). But the system has been running fine since. However, I installed some new software and when the program booted it said that I only has 256 RAM. Advised me to upgrade... I rebooted the whole system and the RAM showed again. First I would like to know if this is a sign that one of the sticks of RAM is bad? Second, if I add more RAM since it's PC2100 I have to add the same RAM right? I read some of the other posts and they say that they don't have to be the same speed, but the system will be restricted to the slower RAM. Also what about Latency, Voltage, and CCS? These are some o the things that I have seen that I dentify RAM. Is this going to make a difference when I purchase the RAM.