I had the same memory problem. I removed all the sticks. Blew compressed air in the slots. Some dust bunnies somehow got in even though I was careful in my original memory stick install. Rebooted and all memory registered. Bottom line is even a little dust in the memory slot can prevent all of them from registering. Also recommend cleaning all contact points of the memory sticks with denatured alcohol and cotton swab. Make sure you are and remain grounded whenever working with memory. VERY important.