I too have a 7mm Mag. Mine is an older Savage model 100. I use remington maybe 150 grain, not sure. Not sure about the ammo because a few years back I got a .308 and if I'm not bow hunting, I am using the .308. Not because the 7mag is not good, but man, it kicks. I think the 7 is perfectly fine weather your shooting the 130 something or 150 something grain. I can't explain your lost deer, but by the sounds of it, it is a freak case. I have shot several deer with my 7 mag and all but one dropped where it stood. the other ran about 30 yards and went down. IMO if you dont mind the recoil, stay with it.
Good luck