This is likely to be due to the fact that the routing service can't route to the start point by the mode of transport selected, as it thinks the road is unsuitable. You would need to select a start point nearby and then extend the start of the chosen route once it has been generated by dragging the start marker to your preferred start point (with the Auto-Plot feature turned on).
When I use the "Make a Route" feature it cannot do it. It seems it is because I start from a main "A" road. If I drag the start point 1/2 mile away in a side road the make a route feature appears to work. Is this a problem for all or am I doing something wrong???