The whole problem could be a connection problem.
Before you try anything drastic: Shut off the computer and unplug it. Then take out all the memory chips. (Mark the one that does work with a small piece of masking tape or something so you'll kow which one it is.) Then use a can of compressed air to blow it out. Then replace the memory chips, making sure they are seated firmly onto the mother board. Connections can get dirty from just setting there. It's a good idea to blow out the whole thing from time to time. Good luck!
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My best,
Ernie

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