you had that problem in SH3 and 4 as well where the sonarman would say "no contacts" when you can plainly hear them.
I always man the sonar myself to doublecheck. Be sure to drop the speed to zero, since in SH5, unlike the previous versions, background noise is modeled. The faster the boat moves, the more background noise increases, drowning out the fainter noises.
you can also look at this, if you want a more talkative soundman:
http://www.subsim.com/radioroom/showthread.php?t=166593
By default, your soundman will only update contacts once every 60 seconds.
If you drop the time for precise contacts to 15 seconds and imprecise contacts to 180 secs (what I use), he will report contacts on a more regular basis.