If you can't auto-plot on a path it is likely that the map provider doesn't have the path tagged as accessible by the auto-plot mode you have selected (On Foot, By Bike, By Road) on their map database, e.g. if you have the Auto-Plot switch set to “By Bike”, the path may not recorded as being accessible for cycling. Changing the Auto-Plot switch to “On Foot” usually overcomes this or alternatively you can temporarily turn off the Auto-Plot switch. Premium members can also try switching between OSM based maps and Google maps, which use a different routing engine.
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.