Change 770 to 700.
Sorry, the 770 is basically a renamed 710, and while there were some good ideas behind that design, it wasn't executed to Remington's traditional level of success.
The problem is probably with the magazine not putting the fresh round in just the right place for the bolt to pick it up when moving forward.
The magazine may not be fully seated in the action.
The spring in the magazine may be weak or obstructed so it isn't lifting the round high enough.
Or the magazine platform may catching on the way up, with the same result. The cause of that could be the platform bent or misaligned or the magazine body dented so it obstructs the platform's movement.
Or the top of the magazine, forming feed lips, may be malformed or bent, again, putting the cartridge at the wrong height or angle or holding on to it too hard when the bolt is trying to grab it.