Google Maps with Offline Maps sometimes gets stuck on "Finding best route"

Google Maps with Offline Maps sometimes gets stuck on "Finding best route"

I don't think it's a GPS issue 'cos my phone's GPS is normally fine and also, as soon as I connect to WiFi it gets past the "finding best route" screen. Also I guess if it can't find a good GPS fix then I doubt it'd say "finding best route". My experience with google maps has been that if it can't get a good fix it still gets to the navigation screen.

And it's definitely not an offline map issue. I have quite a big offline map, not yet expired, and like 600MB covering an area that includes all of london and quite far behond, so (while my sense of direction isn't great), I don't think map coverage is the issue here. Also if I choose a postcode that is definitely outside my offline map, like somewhere in Glasgow, then I get a different message "no results found in your offline areas". The postcode is well within my offline map, and if it wasn't, that wouldn't cause the "finding best route"(and not moving beyond that message), error.