Reshoots are often treated with suspicion by people not in the movie business. They take it as a sign a movie is in serious trouble and desperately needs fixing. But the truth is most major movies build reshoots into their schedule. The studio reviews the footage shot in principal production and find they're missing certain pieces that would make it flow better. This could be as small as adding insert or reaction shots or filming a whole new action scene or subplot. But if the reshoots are pulled off correctly, an audience wont be able to tell the difference.