I'm having the exact opposite problem. A small convoy of 4 unescorted freighters in a 2x2 config were traveling at steady speed and bearing for the entire 6-8 hours I was stalking them to get into a Fast 90 position.
In position and not caring about simultaneous strikes, I waited as the closest ship reached my scope crosshairs and fired two torps.
I think I had accidently changed my AOB slightly, so the two torps were heading for the aft of the ship. The first hit, and the second went behind but hit the lead ship in the next column.
So here I had two freighters hit and on fire, and all 4 of them continue about their course as if nothing happened. No zig-zagging, no speeding up, no evasive maneuvers of any kind.
This has happened every time I've engaged a target or group of targets so far. Granted, this is ~ September of '39, as I had waited to play SHV until after the 1.2 patch, so maybe it's just an "early war" thing.
A couple times I even engaged a lone freighter on the surface where I followed it to an intercept point in sunny clear weather, and it never once changed course or speed. It just proceeded to meet me to it's death.
Is this normal?