In that case it most likely is a system board issue. 
I have a Dell R900 with 4 risers of 32GB each. Initially, I found out that one of the risers had gone bad so I purchased a new one and also more memory. I can verify that each riser works in slots A and B. However, when I try with anything in D and C, I get an error that Riser D cannot pair/train with Riser C. I can take the same two and place them in A and B without issue. So, it would appear that I have a system with only 2 good memory slots. Any hints?  James
Dell R900: Issues with Riser C and D