The GPS module (I'm assuming it's an external one?) works on an NMEA2000 network. The network has to be terminated correctly, and (I think) needs to be powered (the module may have been an exception for the power requirement; don't remember now.)
Your best bet would be to re-read the installation instructions, and make sure you've got everything set up as suggested. It's too variable to throw out suggestions and hope to get it.
Once you're sure it *should* work, call your Lowrance/Navico/Eagle tech support.