HIGH Density Ram is not compatible. Ram is in interleaved pairs.  The white ears are bank 0 and the black ears are bank 1  
Currently at Bios A13, on way to Bios 22.  I see the memory correctly as 2, 2, 4, 4 in System Setup in the info section.  BUT, it only allows the bank of 2s to be used.  Even Diagnostics does not see it.  Why? If seen and identified, it should be usable.  Does this issue go away in one of the higher Bios updates?  Some websites are stating the upper limit is 16 for this board.   By the way Win 10 Pro (1909) works just fine on a 8gig machine.  set the paging file on a USB3.0 32 gig flash drive after installing a 3.0 card.  Ramboost also helps      
Optiplex 755 Memory question