Just chiming in a year later to say I had the same problem, and the same solution.  I have an x399 Fatal1ty Professional Gaming motherboard, Threadripper 2990wx, and 4 x 8GB Kingston HyperX 2400 DDR4 (HX424C15FW2K4/32).  DIMM_B2 (on the far right of the board) wouldn't recognize anything.  Switching out RAM sticks didn't work.  Unplugging the PSU and mashing the Clear CMOS button for ten seconds, then upgrading to BIOS 2.2, then to 3.3, didn't solve the problem.  (My computer would just power cycle continually with 2.2 as it couldn't recognize the Threadripper 2--I had to use the Flashback button and a USB stick without powering on [cool feature!].)  Then I took off the cooler and TIM, raised and lowered the TR4 bracket down as straight as I could, reapplied TIM and cooler, and ë moë! Problem solved!  Thanks! 

So try not to blame hardware as 'faulty' when you're running it above spec.

Anyways, as far as a more 'helpful' comment, 3200 can be difficult to attain, depending on chip quality and board used. I have an MSI X399 that did not play well at any speed above 2933, but the asrock taichi x399 allowed 3333 without a problem using mostly auto settings. Most of it comes down to silicon lottery. Using 4 samsing b-die 3600MHz rated dual rank DIMMs

It seems you misunderstand what I said. I doubt the RAM was faulty in the first place. There seems to be little consensus on what RAM is best suited for this platform.