I don't think you are having a bug here, it is just that RSRD provides historical traffic both in quantity and routes, and if you are away from the hot areas you will not find much to shoot at.
If you want to keep RSRD and its historically reduced targets, I would strongly suggest having a look at the info provided with the mod and try to hunt in spots where ships must get through sooner or later, like the Luzon or Formosa Straits, and of course the japanese coast near the entrances to the inner sea and main harbour cities. Beware that ships might actually -as they did in real life- be hugging close to the coast to have additional protection.
Finally, ensure that you start careers at least in late 1942 so you can have radar. This will ensure that anything in a 50 km circle around you gets detected.
Have fun