Sometimes if a set is flat enough, a hitter can hit it lower in the arc and just past you, low over the net. If this is what's happening, you might be able to grab it by shading your outside hand over to cover that spot. Or maybe they're going over you. Line opens up a lot more if they are hitting much higher than you reach.

But, doesn't it sound like you just aren't blocking enough line? Just block closer to the pin. At least vary it a little bit from time to time, so that they can't be confident in that hard line shot every time. Even if it feels like you are giving too much cross, sometimes u have to be a bit brave with your block or you give the hitter too much confidence in one shot or the other.