Does all 16gb show up in the bios?
If so, it's likely an issue with the operating system.
If not, then reseat the memory modules into their respective dim slots.
If you then still get the same results, try both memory modules separately in each dimm slot to test if the issue is either with the modules or the dimm slot.
The sticks arent running at different speeds, it's just two different XMP profiles you can enable for the sticks.
Have you tried reseating the CPU? Is the CPU cooler on OK?
Yes, the BIOS detects both sticks. What is 2x8? 16.