Perfect!, so it's working now since you moved the 4 GBs memory sticks to the blue slots and installed the 8 GBs memory sticks in the black slots.
You are right, accessing that link:
Desktop Boards — Single- and multi-channel memory modes, you can notice that in the image as an example the smaller memory sticks are in the blue slots and the bigger memory sticks are in the black slots.
Thanks for you feedback!
If you experience a similar issue again and if the memory is properly placed, you can test each memory module individually and ensure the power supply is sufficient for the configuration you are looking for.
Thanks for the response. The sticks are working fine, now.