nroute won't lock on roads for longer routes?

I used your great site to load OSM routable maps into my Mapsource and nroute. The process worked beautifully and the maps look great. My problem is when I hook up my GPS to my computer to try to route (in nroute) it will only calculate the route ‘on roads’ for shorter distances less that 150 miles. Anything further than that and it seems like the calculations hesitates and then it draws a straight line cross country and the voice says something like “drive 1080 miles sw”. I have it set to ‘lock on roads’. Do you have any suggestions as to what I might try. Thanks so much.