Did you look through the binoculars and designate that target for the deck gun to engage? (Space bar when looking at it)
Sometimes that jump-starts them into firing, other times you'll have to go and manually jump on the gun to get the crew to appear on it.
Designating a target does work from time to time though. Accidentally designated a target on my way out of Kiel... German Merchant. They happily fired away even though we weren't in battle station's or anything lol
Took me 5 frantic minutes of searching for a cease fire command before I had to go to Periscope depth to get them to stop
