Diamond ore not appearing anywhere

When I go into new chunks on my survival world and deep into caves, it's really hard to find diamond ore. I've went into 2 big caves and they go down to about diamond height, but no luck finding them. I think there's a bug in the diamond generation because I see all the other ores there should be (coal, iron, gold, copper, redstone). Or maybe this is intentional and the diamond generation is just very rare. I can't decide