How big is this vessel that you're trying to track? I've noticed (on a few occasions) that small vessels travelling slowly in rough seas, and heading toward you with screws behind them (as opposed to broadside or moving away), are sometimes difficult (or even impossible) to pick up on sonar until they're much closer.
__________________
" The journey of a thousand miles begins with a single step."
-Miyamoto Musashi
-------------------------------------------------------
" What is truth?"
-Pontius Pilate
|