When writing scripts, Syd Field says that you should only use flashbacks when there is no other way to give the facts to the audience. Flashbacks should happen in response to a character’s emotional state. They have been triggered by something in the present that reminds them of an extreme emotion in the past.
The rule of only using flashbacks when there is no other way reminds me of a similar rule about dialogue. Screenplays are about telling stories with pictures. Only use dialogue if the information cannot be got across with pictures alone.