sonarman detection impairment ?

[Tozzifan]

Hello,
I'm still on the first patrol of my first career (started december 1941, now around march 1942), and still roaming south of Formosa strait, unsuccessfully hunting

along this timeline, I've detected only three ships, although having done my homework, trying to be compliant with shipping lanes maps found through subsim forum (patrolling slow, from BUngo/Formosa straits, to SIngapore)

of course I've patrolled using time compression 32-128x (frequently asking reports to sonar station)

what troubles me is that all the three ships detection was achieved by myself, occasionally using sonar

what then (a deaf) AI sonarman stands for?
or am I missing something?
or is something in my modded (described in signature) SH4 out of place?
or just a mere lack of luck?

thanks