Garmin advice for a numpty...!

Hi, instruction manual isn't helping, or could be just me! This is the second time it's happened. Selected a round trip course to follow on my Edge Explore, selected "Ride" and then hit the "start" button and all seems good as I follow the course guidance. But then I notice something which happened before, that although I'm on course, the little blue progress line appears to be still at the start point. It looks like a blue elastic band and I realise half way round, that I'm not actually clocking up any mileage! What is this numppty doing wrong? Thanks for any insults in advance..!