K i belived it to be the old bug "When playing as a vampire, http://elderscrolls.wikia.com/wiki/Ebony_Shield_(Skyrim) or http://elderscrolls.wikia.com/wiki/Blades_Shield_(Skyrim) will not appear on vampire characters even when equipped. After curing vampirism, the Ebony Shield and Blade Shields will be visible again. Other shields not affected."
It might have been an older bug, but it became very prevalent with Dawnguard, so we put it on the "discovered bugs" list.