Question RAM not recognised/faulty!

Feb 4, 2021
I recently installed 12gb of RAM (3x4gb) in my motherboard. For about 10 minutes Windows recognised that I had installed 12gb of RAM, but required me to restart the computer. Once I restarted my computer the extra stick of RAM was no longer recognised, meaning Windows only detected 8gb of RAM. Also, using CPU-Z, my memory used to be on double channel, but has now changed to triple channel.
I do not know if all this information is necessary to be given but I may as well provide it all.
So, what could be the issue?
Would it be a stick of RAM that is faulty?
Could it be the DIMM Slots that are faulty?
Any information on any solutions would be greatly appreciated.

PC Specs:
CPU: Intel Core i7 920 @2.67GHz
Mobo: ASUSTek Rampage 3 Extreme
RAM: Corsair Vengeance DDR3 1600MHz
I suspect that your ram sticks are not all from the same matched kit.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.

But,first of all, read your motherboard manual.
It will tell you where to install 3 ram sticks .

If you think there might be a faulty stick, run memtest86 with each installed by itself.

To get mismatched ram to work properly together, you can try to increase the default ram voltage in the bios.