Your most likely problem is that you shot a group and your lens was probably f4 or lower. For group shots using a wide angle lens, f5.6 will typically cover most depending on how they are positioned. f7 or f8 will definitely cover everyone. Are you using 3D tracking, 51 point or single focus? Like what Marcel have mentioned, a sample picture and exif data will help us analyze what the cause is. Normally the problem leads to the user, the users like us who are not familiar with the controls and have very little understanding about the effects of DOF and FL.