Both kits, a stick is having difficulty working.
Not sure why, but I remember that my desktop was booting normally, only to realised that I have only 1x8gigs working and after restarting it return back to 2x8gigs.
Some time later, it does the same thing, however, diagnosing a stick at a time, one of the two refused to be detected.
Fortunately, a friend of mine with the same RAM kit handed over his. Mysteriously, it detects all 4 sticks, and once again, it happened again--some time later.
Both of our kit, one of the stick had a sudden drop out, fortunately, his kit fared slightly better, one of his faulty stick will work at stock, but with XMP, it fails.