When sighting in your hunting rifle, it doesn't take many mistakes to turn the process into a long, laborious and expensive process. Failing to check your bedding and scope mounting screws before starting is a good way to waste a box or so of ammo.
Another bad idea is trying to go cheap and use a mixed lot of old ammo. This proves nothing, as ammo of different brands and bullet weights is almost sure to shoot differently. Even changing lots of the same brand and bullet weight can shift the zero noticeably.
Shooting from an unsteady rest is a poor idea. The more solid your position the better your sighting-in results will be.
If your rifle has recently been cleaned, fire a couple of fouling shots before firing for effect. Don't adjust your sights on the basis of a single shot. Everyone experiences the occasional errant "flyer" that is not a true indicator of the rifle's point of impact. Base adjustments on groups of three or more shots.