Hi,
Were you always using 12GB and one morning Windows decided to change it to 8GB, or has this issue begun after performing hardware changes (for example, adding new ram sticks)?
From my experience this issue would happen if the motherboard managed to identify the sticks but couldn't get them to work at the same time, if I would have to guess, timings or voltage issue. Here's what you can do:
1. Check the timings of all the sticks you currently have (Will be written on each stick like in the image you showed, or go to CPU-Z and it can show it for you). Write them down, and check the highest timings for the lowest frequency, also make sure they have the same voltage. Go to the BIOS and update the settings for all the sticks - highest memory timing, lowest frequency, and the working voltage (probably 1.5V for DDR3, can't be sure though). Save, reboot and check.
2. You may need to up the memory controller voltage - but I don't have any typical or safe values. Start with small steps, if it doesn't work maybe someone else can add more insight about it.
3. If neither of those things work, it could be something on the hardware level and not the BIOS level, in that case not sure what else you can try - might be the motherboard, might be the sticks. My Z68 doesn't like having all slots filled, not sure why. Some memories don't work well with one another as well.
