I recently workshopped a student piece. It opened with a scene of the main character on her way somewhere, then went into flashback for the rest of the chapter going over the previous 24 hours. It was well written, but the tension was lost - we already knew the character was going to get in the car and head off for her destination, because the writer had said so at the beginning. It's like telling the punchline before you do the rest of the joke. It doesn't work.
Worse are those flashbacks where the character settles down to have a jolly good reminiscence like some great-uncle at Christmas beginning with 'I remember those lovely holidays we used to have at Gran's...' For some reason, they're often inspired by train journeys and as the character drops off into a dream of times past, so do we.
Of course there are exceptions. Donna Tartt's The Secret History gains a lot of tension because we know from the start that a main character is going to be murdered by one of the others. But we don't know who murders whom until the end. Anita Shreeve's The Pilot's Wife uses flashback to unravel the story, but each flashback section advances the plot with information the reader didn't know until that moment. So flashback can work, but the chances are more likely that they're losing tension and pace. Don't do it!