That's the reason why it's important to have the speed/cadence sensor. GPS reception doesn't exist in a tunnel or under a bridge, and can be very weak under heavy tree cover especially at startup.

I had to experience the problem while accompanying a boy scout troop during a 50 miles outting. We were at the Avenue of the Giants in Northern CA under the redwoods. The GPS (305) couldn't pick up satellite connections where we started the ride at the campground because of the trees, I needed to wait for a meadow for it to start. Once GPS connection started it did pickup the rest of the ride even under heavy tree coverage. The Speed/Cadence sensor did however record the complete ride from start to finish so the boys could prove they had ridden 50 miles.