Your problem is that your probably using a higher end bonded bullet, NAB, or a NPT, or TBBC, or something of that flavor. I know what you are talking about, I filmed a buddies white tail hunt last year with the similar results. He shot a WT with a 7RM from 255 yds and it penciled both lungs, perfect shot placement. The deer stood around, walked, and finally laid down 2 minutes later to expire. I am sure this animal could have covered a couple hundred yds in this condition.
when we cleaned the animal both lungs had been hit with holes the size of nickles in both. He was using TBBC.
I would say keep your rifle, but buy softer bullets, pick up some fed blue box 150's or some Win Super X, they will die nearly instantly I promise.
Harder premium bullets don't open up well on soft light game, they are meant to hit moose and elk shoulders.
PM me if you want the video.
Mike.