I had the same problem (didn't recognize one of the aftermarket [Samsung] RAM cards) and tried 'reinstalling' the cards with no luck. As a last effort I tried doing the diagnostic (holding D at startup) which reported no problems - but when it booted up it recognized the other card!!
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(it’s at least easier than taking the machine apart again; and also may just be a good idea to do any time you install new RAM to force the machine to recognize it?)
I had the same problem (didn't recognize one of the aftermarket [Samsung] RAM cards) and tried 'reinstalling' the cards with no luck. As a last effort I tried doing the diagnostic (holding D at startup) which reported no problems - but when it booted up it recognized the other card!!