ASUS x79 BE

Have a question, have 16 gig memory installed, bios see all 4 sticks, but when it boots up (boot screen) says only 12 gig, OS said 12 gig. CPUZ said 16 gig triple channel. Just wonder if any one has any suggestions. Also have changed out memory same issue.

Your sure you didn't mix one 2GB stick in ? (I know, I'm ducking)I would pull the sticks till I found the one thats being read as 2GB or possibly that the RAM slot is only reading a 4GB stick as 2GB. It will at least narrow the issue down.

If it turns out to be the module...hopefully you can get a warranty replacement(If its indeed labeled as 4GB) If its a Ram slot, try moving the modules around (If you haven't already) They may just need to be seated better.

What does a Memtest show them as 12GB or 16GB?

Common issue - BIOS sees 12GB, system sees 16GB but it's running in triple channel just because in real 3 memory sticks are working. I had the same on X79 and now on X99 too.
Clean memory pins, try to clean memory slots ( condensed air or something ), check if memory sub timings aren't too tight. Reseat memory 2-3 times. Can also try to bump memory voltage.