As others have reported, you can go for days without seeing any enemy units.
Or complete your objective(ie patrol grid for 24 hours) and then sail home to refit without seeing a single enemy unit.
Thank god for dynamic campaigns
It is not unusual to experience what you are experiencing.
How long have you patrolled in these instances?
If you want enemy units at a great amount, i suggest starting a 1944 or 1945 campaign, i guarantee you you will have your enemy units then
