Because flashback is in the past it's like going backwards, whereas stories should be all about moving forwards. If the story starts with the narrator talking about how they once drove over a cliff, we know they survived it otherwise how would they be there talking about how they once drove over a cliff? So the tension is lost, unless the flashback is adding something we couldn't know from the present narrative. In other words:
What works is when the flashback scene illuminates something in the present. We gain information from the flashback scene that we couldn't get in any other way and it moves the present day story on.
What doesn't work is when the flashback scene is just explaining. The scene where you explain why your character is in a bad mood. The scene where you explain why your character has fond memories of their grandmother.
The non-working flashback scenes often work like this....
a) The character is staring at themselves in a mirror one morning. They think back to last night - cue flashback with them walking into a bar the night before. The flashback happens - the night doesn't go well. The scene changes back to the present. The character stares at themselves in the mirror. The End.
b) The character is on a train. They think about their grandmother. Their eyes close - cue flashback. They are a child on a beach with their grandmother. They have a nice time. The flashback ends and the character wakes up in the present. They arrive at the station. The End.
These flashbacks (and I have seen many variations of them) don't work because nothing in the flashback illuminates stuff that's happening in the present.
What would have worked better is:
a) Start the story with walking into the bar. Whatever happens in the bar then happens in the story present. It happens. The End. This is putting the story into the narrative present.
b) The character arrives at the station and is met by their grandmother. As they talk, the character realises that grandma seems smaller, more frail than how she was on the beach all those years ago. They give grandmother a hug - they still love her. The End. This is putting the flashback into backstory, ie the information is woven into the narrative present.
Some of my favourite writers use flashback brilliantly. But lots of beginner writers don't, so be careful. Make sure that if you must have flashback it really is adding something to the story that couldn't be added in any other way.