[UKönig]

I think this has a simple explanation. Your watch crew is not experienced, and so, they can't ID the ships or see them at range properly. It isn't until the contact gets to within eyesight on a clear day that they can follow the ships. I have been damaged (killed once too) because a watch crew was too slow in detecting things.
The normal watch compliment is 5. Two seamen, two petty officers (watch certified) and one Officer (also watch certified). The four crew keep a look out, 1 in each 90 degree sector, with the Officer (or captain) making periodic checks
in all sectors. When aircraft became more of a threat, two of those watch crew would have to start scanning the skies as well.
And the visual range from the 'bridge' of a U-boat is only about 16Km, even on a good day...