I have had the same thing happen to me. I borrowed a 30-06 gun and ammo. Put two shots within a 1" of each other at 200 yards at a cow. They kept saying I was missing because they saw the bullets hit the ground on the hill behind her. Double lunged her. Looked like she was never hit. After about a minute she finally fell over. She kept eating the whole time.
Here's where the hate mail will come in. Speed doesn't kill, energy does. It was too light of a load.
That doesn't seem too extreme. I hit a cow once that stood for several minutes. Since I wasn't sure if I hit her the first time, I waited for her to get broadside again since she turned directly away from me after the first shot. She was uphill from me and I really didn't want to shoot her directly in the hind quarters. After several minutes of just standing there, she turned enough for another shot. When I inspected the wounds, I found that the first shot hit her in the boiler room exactly where I was aiming. The exit wound was small though. The second shot went through both shoulders and stopped at the hide. I was using basic soft-point .270 ammo.
Not crazy, they are just that tough. My daughter shot a cow at just under 200 yards with a .243 and hit perfectly in the boiler room. The cow lurched and fell and then got back up and struggled and fell and got back up, etc for probably 10 minutes until my daughter couldn't stand the noise the thing was making and put another round behind it's head at close range. Another guy I was hunting with shot a cow with a 30-06 at 50 yards. She just walked away like nothing happened so he thought he had missed somehow and shot another one. She did the same thing. We found both of them dead at about 100 yds from where he shot.