Shoe.

Look at the posted picture of the play. (Thx Coco)

Martez Wilson is clearly being held back from making what would be a more-or-less routine run stop.

Curtis Lofton (#50) also has someone holding his shoulder pad, you can see him leaning backwards because of it. His hips are going one direction, his shoulders another.

Patrice Robinson missed the tackle (I think that is who is off Charles' left shoulder) and then got outrun.

Roman Harper almost caught up to Charles, but the dude is FAST.

Other than those people, based on the picture, I'll ask you to educate me; exactly who are you referring to when you say, "In position to make the tackle and didn't?"