Reshoots are an inevitable part of filmmaking. Most blockbuster releases require around $10 million's worth to iron out plot creases, give the movie a few tonal tweaks or fix issues that didn't sit well with test audiences.
Some films need more additional scenes to be captured than others and there's always a stigma attached to projects like this. When extensive patching-up is deemed necessary, it's usually a sign that the production is in trouble.
Adding reshoots to the mix is a fine art, especially when they require actors to get back into character to lay down new scenes. The stars may have moved onto preparing for other roles by then and altered their physical appearance as a result.
Variables like this are what makes shooting extra footage after the fact so challenging. If your star has gained several pounds or shaved his head since filming their initial scenes, continuity goes out of the window when you shoot some more.
There are ways around this, of course, from shrewd camera angles to digital editing techniques, but filmmakers don't always paper over the cracks seamlessly, and there are some high-profile examples of movies where the reshoots stood out like bad CGI in a Star Wars special edition.