Our Auto-Plot feature fills in the gaps for you every time you click on the map. It does this by calculating a route from the last point you plotted, based on your chosen Auto-Plot option.
If you find that you are unable to auto-plot on a road or path you see on the map, it usually means that the route engine couldn't find a viable route between the last point you plotted (the origin) and the one you are trying to plot (the destination). Route calculations are made using OpenStreetMap (OSM) maps, so this problem means that one or more of the origin, the destination or an unavoidable road in between are not tagged on OSM as suitable for the Auto-Plot mode you chose (BY ROAD, ON FOOT or BY BIKE).
To overcome this you can temporarily change the Auto-Plot switch to another setting (ON FOOT usually has the widest access) or turn it off to plot manually. Deselecting any of the Auto-Plot sub-options (PREFER OFF-ROAD, AVOID UNPAVED and AVOID HIGHWAYS) can also sometimes help, as these options by nature add restrictions.
You can also contribute to improving the OSM maps by adding missing access tags, as OSM is a community project where anyone can make edits. See this How-To Guide on correcting map errors for more details on this. Changes you make on OSM will flow through to plotaroute.com as and when we implement regular updates, which usually take place on a monthly basis.