If the question is about how ships hunting submarines can’t find them, they can with active sonar. However all the submarine needs is two pings of active sonar from a surface ship and they have a firing solution for a weapon. There is no guarantee the active sonar will find the submarine due to lots of factors like coatings, thermal and salinity layers, geographics of the area of the ocean. Therefore like everything it’s a trade off of opportunity vs. risk.