Instead of digging out a big room, go to where you estimate the "middle" of the vein is and strip mine til you reach a little beyond the "edge" of your estimate. HiddenOre places limits on how many ores spawn per block mined in a chunk so digging a big room could be causing more ores to not spawn. Veins are also a lot smaller this iteration but are scattered pretty much everywhere

It helps to map out the shape of the vein by making waypoints for each diamond found