A flashback stops your movie. That is not criticism, it is just the mechanics.

The present-tense story goes on pause, the audience holds the tension in one hand, and you ask them to invest in a new scene. That's an expensive ask, and most flashbacks never pay it back.

The failed version explains. Your character behaves strangely, so you cut back to show why, and the audience collects a fact. Facts are the cheapest thing a scene can deliver. The version that works doesn't explain, it reverses. It takes something the audience already watched and makes it mean the opposite.

The Godfather Part II is built on that principle. Francis Ford Coppola and Mario Puzo don't cut to young Vito to explain where Michael's ruthlessness came from. They cut to Vito building a family so we watch Michael dismantle one in the same breath. The past scene isn't background. It's the counterargument. Every time we come back to the present, Michael has lost more of what Vito was gaining.

Kenneth Lonergan does it with a single memory in Manchester by the Sea. You spend the first act reading Lee as cold and difficult, and you judge him without noticing. Then the fire. Everything you decided in those forty minutes reverses at once, and Lonergan never says a word about it afterward. He doesn't have to. You're the one who was wrong.

So before you write the flashback, answer one thing. What does the audience believe right now that this scene is going to take away from them. If the answer is nothing, you're writing a footnote, and a footnote belongs in a line of dialogue or nowhere.

A flashback that only informs is a flashback you can cut.