Guy in my Tuesday writers group said my prompt was doing too much work for the reader
At the library on 9th street he read my one line prompt about a diver finding a note in a sunken mailbox and told me I had already written the story, so there was nothing left for anyone to explore. He said a good prompt is a locked door with one key missing, not a room with the lights already on. Do you hand people a whole plot or just a weird doorway and let them wander?