Memory Riser B Empty

I have 8gb of RAM (8 x 1gb modules, 1gb for each slot) in my January 2008 Mac Pro. I have been running this configuration for a year now with no problems.

I noticed today that "About This Mac" says I only have 4gb RAM... I look a little deeper and EVERY slot on Memory Riser B comes up as empty.

So I check the riser, pull it out and reseat it... Nothing. Still only 4gb

Faulty Memory Riser?

 

Do you mean switch the ram from one riser to the other?

Is there anything in the way of electronics that prevents one from putting the B riser in the A slot?