Supposedly, reshoots are a normal part of moviemaking. Oftentimes they’re done to make the story flow better, or to add something that focus groups shown the movie in pre-release screenings thought would somehow sharpen its focus or to address a weakness.
Though reshoots can also happen when a movie is believed to be in danger and the studio is doing their best to salvage something from their investment of both time and money.
The most blatant example I recall is from Exorcist: The Beginning (2014).
Which is exactly what the studio wanted (which begs the question: Why they hired Schrader in the first place, seeing that he was never known for movies of that type).
More recently, while Fox did not replace Josh Trank at the helm of Fantastic Four (2015)–perhaps they should have–producer Simon Kinberg did go in and reshoot certain scenes when it was clear that Trask was not working out.
Take your pick.