When viewing the map, press the Map button. You should get a window in the top left corner. Tap the title and you get a drop down of options like Overview, Route, etc. GPS Viewer is the one you want. It will display the number of satellites that the RNS510 can see.
And I mean leave the car out in the open with the RNS on. In fact, go for a drive if you want. Once it's found 5+ satellites, it'll get a proper lock. It occasionally happens to me but not so frequently as it once did.