I suspect it’s an open circuit or you have a gps issue. I have a newer model Garmin, so I’m not familiar with the 480. I’m assuming that there is a page that shows you what satellites you are synced. Until you sync with the appropriate number of satellites, it won’t output all the data. Take a look at all your diagnostics on the 480. Make sure the gps is working properly first.
Garmin’s gps antennas are active devices. They have been known to go bad. I had one fail after about three years of use. The diagnostics should be able to validate if you have an antenna issue or some other problem.