Here's why:
* The scene is chaotic: The Cornucopia is a scene of utter chaos and violence as all tributes fight for supplies.
* Multiple tributes are firing: It's likely that multiple tributes were firing at the Cornucopia, and it's hard to tell who specifically shot at Katniss.
* Katniss doesn't know: Katniss herself doesn't know who shot at her, and she focuses on survival rather than trying to identify her attackers.
It's also worth noting:
* Glimmer and Marvel: Glimmer and Marvel, tributes from District 1, are shown firing arrows at the Cornucopia, but they don't target Katniss specifically.
* Cato: Cato, the tribute from District 2, is the one who eventually corners Katniss at the Cornucopia, but it's not clear if he was the one who shot at her earlier.
The point of the scene is to show the brutality of the Games, not to pinpoint individual attackers. Katniss survives by her wits and cunning, not by knowing the identity of every person who shoots at her.