This is a tough one, but it sounds like the velocity of that H870 load is a lot lower than you should be getting, so the drop is a lot more than it should be at that range.

I think you need to chronograph the ammo to see how fast it is going, and work from there. H870 is a pretty slow powder, so I would think it would be easy to get a load that is moving too slow for long-range work.