Hi there,
Let's see. There are so many Nuvi models that it is hard to know exactly, but the mode used for live tracking is important. Your GPS may or may not support one or both. The two are NMEA and Garmin's PVT protocol. TopoFusion supports both, but it won't look for both. You'll need to try them both. It's a matter of getting the unit into live tracking mode sometimes, as well, but often that is automatic.
So if you tried Garmin mode, try NMEA, or the other way around, and let me know how it goes.
Thanks!