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You could try reseating the ram. If that doesn't work you could try moving one of the known working sticks into those slots. But i would get on the phone to apple if reseating doesn't work.
 
You could try reseating the ram.

Agreed. I'd say most problems with RAM not being recognized is because it isn't fully seated in the slot. If reseating doesn't fix the problem, you might have defective RAM....try to find a RAM tester to see.
 
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