It is because you are using Ram sticks of different sizes you NEVER mix and match ram. If you want to use both you have to purchase either a second 1gig stick or a second 2 gig stick. They will not work properly if you have a 1gig and 2 gig stick at the same time.
Dead wrong. Mixing and maching RAM can fail, but it usually doesn't if the modules are very similar. These modules are identical except for the capacity and capacity differences usually aren't a problem.
I had a machine with two 1GB modules and 2 256MB modules for 2.5GB of RAM for years, no problems whatsoever. I also had another with 3.25GB of RAM and another with 1.25GB.
Sometimes mixing different kits fails, but with identical modules it usually works with DDR2 and DDR3. I don't recommend it if you can avoid it easily, but that 2GB module is dead if it fails even without the 1GB module in the system.
Problems related with mixing are usually different densities, voltages, and timings.
This 2GB module is undoubtedly dead, not having compatibility problems.