You need to inspect your RAM sticks and verify their size. Power off and unplug from the wall. Open the side panel and touch bare metal of the case interior to discharge any static in your body. Then carefully remove each stick. There likely will be small levers, like the white ones seen in
this image, to push that will unlock and push out the sticks. The sticks should be labeled to indicate their size. You need to verify you do indeed have 8GB.
Return each stick back to the same slot. Note they can only go in one way as there will be a notch or key in the stick you align to a corresponding key in the slot. Once aligned properly, push straight down on each end of the stick until that lever "clicks" into its locked position.
Note in some cases, if the sticks were not properly seated before, removing and replacing them in this manner will scrape clean their contacts (a good thing) and ensure they are properly seated.
Do NOT touch the electrical contacts on the RAM sticks.
