[SOLVED] POST fails only when second dimm installed ASUS TUF X299 Mark 2

[DBarryM]

Upgraded my desktop with new ASUS TUF X299 Mark 2 mobo, CPU, and DRAM. POST will complete if either of 2 dimms installed individually, but POST fails if both dimms installed. Details: CPU Intel i9 9820x Skylake 10 core 3.3 GHz, DRAM Corsair DDR4 Vengeance LPX 2666 32 GB (2 x 16 GB), Cooler Master ML120L.
If one dimm is installed in A1 slot, POST works and Win 10 boots. If two dimms installed in A1 and C1 slots (recommended by ASUS) POST fails. Attempted solutions: pressed MemOK button, POST tries tuning through 3 stages, then fails. Tried EZ tuning to overclock, no change. Updated BIOS from 1503 to 1704. Tested both dimms by installing each one in A1, both work ok individually. Installed dimms in A1 and A2, or A1 and B2, no success.Made sure XMP was enabled. Checked DRAM frequency and timings, frequency shows 1600.
I can use PC with 16 GB, but am obviously perplexed by not getting 32 GB. And yes, I checked that Corsair DRAM was on the ASUS QVL list (ver 4.31 listed, my package shows no ver number).

 

[helpstar]

Did you check the RAM individually with a memtest application like www.memtest.org (the usb autoinstaller)?

reinstall the RAM in DIMM slot (counted from left to right) 3 and 6

any QLEDs on while trying to post?

Reset the BIOS by using the jumper clrRTC

try installing into Dimms C1 and D1 (6 and 8)

How did you update the BIOS? Reflash in BIOS itself with a USB flash drive and the flash menu within the BIOS

Check the Pins of the mainboard´s CPU socket. Any bend pins like:

View: https://www.youtube.com/watch?v=buHK_fy9xxY

 

[DMAN999]

Make SURE to put them in the correct slots (A1 and C1 for 6 core CPUs or C1 and D1 for 4 core CPUs) for your MB as shown on pages 1-5 and 1-6 of your MB manual:
https://dlcdnets.asus.com/pub/ASUS/mb/LGA2066/TUF_X299_MARK2/E12906_TUF_X299_MARK2_UM_WEB.pdf

 

[DBarryM]

Thank you both (Helpstar and dman999) for your quick suggestions. Helpstar, most of your suggestions I had tried, and same with your point, DMan. But the Helpstar suggestion about installing the dimms in C1 and D1 led me to the solution. When I put them in C1/D1, D1 showed up in the BIOS but not C1 after the POST completed. Aha! says I, and moved the C1 dimm back to A1, and everything worked perfectly with dimms in A1 and D1. ASUS recommends A1 and C1 in the manual (slots 3 and 6). Hope this helps others.
So, either slot C1 is defective, or will not co-work with a dimm in A1.