Hi,
My Blackbird came with 16GB (Samsung) on the back slot (relative to the user seating in front of the computer). I've bought another 16GB from Crucial (CT16G4RFD8266 16Gb Ddr4-2666 Rdimm, 16 Gb (1 X 16 Gb), Ddr4 Sdram, 2666 Mhz Ddr4-2666/Pc4-21300, 1.20 V, Ecc, Registered, 288-Pin, Dimm) installed it and it worked for a few days.
Then it stopped worked and I've tried all combination front/rear slot samsung/crucial and found that both sticks work on the front slot but not on te rear slot, giving me only 16GB. Neither work on the back slot alone and I suspect the back slot is not working or there is some firmware issue that was triggered when I added the additional stick on the back slot.
I opened a support ticket on Raptor (my machine is brand new and came with a defective power supply, which took a month to be replaced). It seems your issue is similar to the one I'm experiencing.
Have you opened a ticket with Raptor?
Regards