Problem: Board Z390i - Only 1 DIMM recognized ... will *not* boot with second DIMM installed

CPU: i5 9600k

RAM: RAM G.SKILL TridentZ Series 16GB (2 x 8GB) 288-Pin DDR4 4000 (PC4 32000) Model F4-4000C19D-16GTZ

Each works in DIMM slot 1, but, machine will *not* boot with second DIMM installed in slot 2.

Updated the BIOS... to 2417 (6/3/2019) ...still not working.

Not running terribly with one DIMM... went ahead with stock (out-of-the-box) OC anyway ...5gHz on cpu & 4300mHz on ram. *Totally Stable* during 4 hr. bench test.

Would be nice to have 16GB of ram though... any thoughts???

thanks in advance

dennis