Flashbacks are great when they are used judiciously. Too many, or of they are too long, can confuse the reader. It also needs to be clear exactly when the flashback begins, and when it ends, with the reader safely returned to the main story. Like any other trick we writers have up our sleeve, that lures readers deep into our character's lives, flashbacks have to be woven seamlessly into the fabric of the plot, They need to appear as a natural progression of the story. Perhaps best left to experienced writers?