How long have you waited for a signal?
I've waited ages - as long as 30 mins sometimes. Wait for 10 mins if it doesn't get a signal, unplug it then plug it in again.
Road Angels are notorious for taking a long time to pick up a signal.
Another way to do it to see if the unit is faulty, is plug RA in and put it on the roof of your car so it can pick up a clear signal, you might still have to wait 15 mins or more - when and if it does, then put it back in your car.