need help in mirror modifier

hello everyone, i am a new bee to learn blender, but i am now having a problem which brought me to this wonderful community. i have improted a mesh(an armor) from a game, i wanted to make it symmetrical, so i deleted half in edit mode, and appled mirror modifier on it, everything went smoothly, but when i exported the mesh back to the game, the mirrored part deformed and mess up. so i recheck the mesh in weight mode and found that the weight for each vertex gourp had been mirrored too, so i wonder if is it the reason that caused the problem? any advise to solve this problem? or if there is any steps i have missed?

it could be that, or maybe you need to recalculate normals ( select all in edit mode and press ctrl N )

thanks for your help, but recalculation didn’t work, so it seemed i should repaint the weight? or any simply ways to copy them?