Planned route problems

[Zbd]

I plan my routes on the plan.tomtom website and then Sync with Device. It does indeed sync to my Rider 550 but part way along the route, it suddenly says 'turn around when possible ' or shows the blue line ending. I then have to stop the route, open it again and choose the nearest point option. It tells me to backtrack and then picks up the planned route again a few miles later.
I know that I haven't screwed up following the directions as I've been using the device for ages. It's only done this on the last few rides I've done. When I open the route, everything looks fine.
Any ideas?

 

[Willy875]

Check the navigation settings on the GPS and on TomTom Maps; the settings must be the same.

 

[Zbd]

What settings do you mean? I haven't changed anything. It has only done this each of the past three routes I've planned and the ridden. I've never had this problem with it before.
I'm due to lead some rides and really need to be able to rely on it!