Fire in the Hole Mission

Hi, I have to complete the ''Fire in the Hole'' mission, which consists of shooting 7 grenades and injure the grenadier without killing him, but no matter what i do, it doesn't work. Whether I injure the grenadier first and shoot the grenade last or vice versa, it doesn't show in the progression bar. Is that a bug in the game or is it me doing the mission wrong? Thanks !