Hi JasonHoffman,
Thanks for the response, and sorry for the slow reply. I ordered my NUC D54250WYK from Amazon, so I went ahead and got a replacement, as I believed it might be a hardware problem. As it turns out, the second NUC behaved exactly the same way. The RAM worked fine when just one stick was in the upper slot, but any time the lower slot was occupied (regardless of whether there was a stick of RAM in the upper slot) it would not boot and the power light flashed 3 times.
I found another forum thread - NUC D54250WYK Dual Channel 1866Mhz memory - that address this problem specifically.
The RAM i had ordered was: Crucial Ballistix Sport SODIMM 8GB Single DDR3 1866 MT/s (PC3-12800) CL10 @1.35V 204-Pin Memory BLS8G3N18AES4 at Amazon.…
which, according to Intel in the thread I cite above, is not supported: "Please bear in mind this motherboard model does not support 1866 MHz memory. So, Intel will not release BIOS updates that support your memory. Your board can handle DDR3L 1333/1600, 1.35V"
Thanks to Amazon's 30-day no-questions-asked return policy, I'm just shipping back the 1866 MHz RAM and replacing it with Crucial Ballistix Sport SODIMM 8GB Single DDR3 1600 MT/s (PC3-12800) CL9 @1.35V 204-Pin Memory BLS8G3N169ES4 at Amazon.c… Hopefully that will work.
To answer your question about BIOS, I was running WYLPT10H.86A.0021.2013.1017.1606, but updated BIOS to WYLPT10H.86A.0027.2014.0710.1904. The RAM still didn't work, as the other forum thread says it won't.