I've been raising and hunting the dogs I raise for a pretty long time. Someone posted the video below on this site a while back, and I have to admit had I seen this years ago I could have saved myself a lot of frustration. For the most part, I agree 100% with what this guy says.
My best advice for someone trying to get a dog to hunt deep is get a pup bred that way and just don't screw it up. Easy to shorten a dogs natural range...sometimes without even realizing what your doing.
https://www.youtube.com/watch?v=eGkdlpIxWbY