Maybe the computer can't run at 2400? The memory can, but is only being used at the speed the computer can do?
If you go to Crucial's web site you can usually find your computer model. Crucial will tell you the memory that's compatible with your computer. That might suggest something about the memory you have now. You probably know this: if you're adding a module to an unused slot, it should be the same. You might not get dual-channel performance if they differ in some ways. How sensitive might vary from one computer model to another. I don't know much about these things. I try to get the same part number. Or, same speed, rank, density (x4, x8, x16), etc. Getting the exact same module is obviously the best option to ensure they play well together. But, it's probably ok to mix modules if their spec'ed similarly. How similar it has to be probably varies from one computer to another.
