What game/mod version are you running?
My best guess is that your display settings are preventing you from seeing the contacts that are there, although that's usually a bigger problem with the Seawolf than the 688I. But if you haven't made sure your brightness/gamma is high enough before, it's a good place to start.
If you're running LWAMI, it's possible that the enemy subs are in the surface duct but are running their towed arrays below the layer. If you were below the layer they would be able to detect you but you'd never hear them unless you went shallow.
It's also possible that you're just running too fast. Most of the opposing subs in this mission have Pelamida towed arrays, so speed discipline is essential for getting the first shot.
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