OS are you running ??
I have a computer with 6 ram memory slots on there, I had 4 2 gigs and just recently bought 2 4 gigs. My motherboard can handle 24 gigs. I installed the two 4s now my computer won't start with all 6 in nor will it read more then 2 gigs of ram in a certain slot regardless of which of the 4 2 gigs it is.