It sucks, but I do think it's just random. I was after the Crimson Vest for SMN last week. Ran it 13x times and only saw it once (lost the lot, of course). 6/13 of those times the last chest dropped the 'Crown of Light' for tanks. No joke. Ran it the first time this week and saw it (also won it somehow with 2 other BLMs lotting against me).

It's just Murphy's Law that the item/s you're looking for to be the hardest to come by sometimes.