Honestly those QVL RAM kits are only a suggestion that the RAM may work at the rated speed. In any case, the issue might be the motherboard. I imagine the Asus Prime series is relatively poor for memory overclocking. That being said, 6400 is a modest OC. If you enable XMP and then manually set the clock to 6000, is it stable? You need to use a dedicated tool to verify stability like the ones mentioned in the post above or Memtest86+