I have only done this a couple of times, so I'm not sure I can explain it. Have you searched MB forum? They are pretty good about explaining stuff like this.

I have used courses a couple of times, once on my first century, once shortly before that on a 70 mile route through unfamiliar territory. It did tell me when I got off-route. You can also build a route where it will cue you when to turn, etc. On the 70 mile one, it worked well even though I had to guesstimate for a few of miles of bike path. On the century route, I was riding with several and we were not sure if we had made a good turn or were off-route. I went with what the unit said and it was right.

Having said that, there are a couple of bug issues. I had to re-set mine last week. If the wall unit is defective, call customer support. From what I've read on the MB forum, they are very good about replacing defective stuff.