Well, I'm not that well informed myself. I'm mostly speculating and may not be helping at all.
What I was saying is I THINK you can hear ships beyond what your hydro guy can.
You are looking at it this way: 'I hear the sound. Why can't he?'
But the game is set up so he only reports contacts when it is realistic for him to do so.
The game has a rendering range. That is: a 32KM circle around your U-Boat. Any thing beyond that circle isn't being rendered by the game.
Example: If you are 100KM away from a convoy - as you are playing the game that convoy isn't really there. The game isn't 'rendering' those ships.
Those ships don't appear in the game until you move to within around 32,000m. (Which is why, on a slow system like mine, the game will pause whenever I come that close to a convoy. The game is 'rendering' the convoy)
If you think of all the ships in any port, and then think of all the ports in the game - obviously the game isn't rendering all those 100s upon 100s of ships. They don't really appear in the game until - as said - you are within 32KM of them.
So, when a ship is rendered - when it moves to within 32,000m of you, it appears in the game. The game is now rendering that ship.
If you listen to the hydro yourself you may be able to hear that ship. Because it is there making noise.
But it's not realistic for you to hear a ship that far away.
To help you understand this, another example of the rendering range:
If you are looking through your UZO or periscope you can lock onto a ship - and identify, get it's speed and course if you use WO Assist - from almost 32,000m away.
This is totally unrealistic. But again, the ship is there, so if you lock your scope at it the game recognizes it. From 32KM away!
Now obviously, your watch crew isn't going to spot this ship until it is more realistic. Maybe only 8KM away.
But if you want to you can spot a ship from almost 4 times that distance. You won't actually see anything, but your scope will lock onto it.
I THINK it's the same with the hydrophone. You can detect things well beyond what your hydro guy can - even to the point where it isn't realisticly possible to do so.
Does that make any sense?
The funny thing is that may not be your problem at all. Like I said it may be that the ship is in one of your hydrophone's 'blind spots'. Blind spots are anywhere directly in front or directly behind your sub - no matter the distance.
You can hear screws in these blind spots if you listen yourself - even though it is unrealistic to do so.
Or it maybe something else. Maybe bad weather, or bad luck, or tired crew, etc.
Just remember that your hydro guys - for the most part - respond when it's realistic for them to do so. Just like the example I gave with spotting a ship with a scope. Just because you can "see" a ship from 32,000m away don't expect your watch crew to spot it until it's realistic for them to do so.
If you want them to hear everything you do then switch off "Realistic Sensors" as someone suggested.
Hope that helps or if I'm totally wrong about stuff someone corrects me