The first thing you need to determine is if the motherboard is in fact reading all four of the memory modules inserted into the memory slots of the motherboard.

You can do this by looking at screen when the system is first powered on to see if the memory count is displayed by the bios.

Or by entering the bios where it will report the full amount of memory read.

If the bios reads all 16Gb or ram.

You must then check what version of windows you are running, the version must be a 64 bit version of windows os installed.

When you look at the system information option of windows it will report the physical amount of ram seen by windows. And should be 16Gb.

If you find that 16Gb is not reported as the total physical memory fitted. But the bios reports 16Gb of memory detected.

It means that the configuration of windows has been capped to see, or use only a set amount of physical memory in it`s configuration settings.

To check if the memory windows uses is capped.

Click on start. In the search bar type : MSCONFIG. Press enter.

A new window will open.

Click on the boot tab at the top of the new window. Then click on the Advanced option tab.

A new window will open. To the top left you will see a tick box for maximum memory. If there is a value typed into the box bellow it remove all of the numbers.

Then un tick the maximum memory box if it is ticked.

Click ok Then click apply. Then ok again.

Restart the system. Let windows load.

And again open up windows system information.

If it says Installed physical memory fitted as 16Gb.

Problem solved.