Don't feel bad it HAPPENS
You tracked it for 6 hours without success you did what you could. Something will eat your Deer, that is the way it is. You may have hit it to far back, or in a leg. They will live a long time gutshot or often will run/walk fine on one front leg. You might have noticed wierd hoof prints if you had snow on the ground to assist you. One argument that I have heard for the big magnums is that they do more damage and this happens less with them. I would agree if the shooter has the ability to shoot as well with them compaired to a standard cartridge. I always worry that a shooter doesn't like practicing with the magnums, and a bad shot is more likely do to less practice at the range.