8 "Unanswered Questions" From Avengers: Endgame, Explained
1. Why Can't Natasha And Gamora Be Brought Back Using The Gauntlet Again?
The rules of the soul stone dictate that a soul must be exchanged for a soul. "An everlasting exchange." Considering uniting the stones is what makes bringing people back from the dead possible, it can be assumed that the rules of one of the stones itself would be a non-negotiable rule standing in the way of bringing those two back specifically. As screenwriter Stephen McFeely explained, "you bring her back, you lose the stone."
In terms of Captain America returning the soul stone and Red Skull putting it back in its place potentially bringing Natasha back, Joe Russo stated, "Even if you have returned it to its original location, you wouldn't be able to get the person back... the tribute soul for the soul stone will forever be sealed in that place, therefore Black Widow is gone forever."
Some may insist that Gamora was brought back from the snap, but this is incorrect. The Gamora currently alive in the MCU canon timeline was transported there from an alternate timeline where she was not killed by Thanos for the stone.