I am trying to use the "Split selected features with selected line from another layer" tool from the plugin Digitizing Tools in QGIS. Sometimes, this tool works with my shapefiles but sometimes it won't for reasons I can't figure out. So far, my troubleshooting skills have led me to check:
a) the projection for the project, the polygon and the polyline is all the same
b) the polyline extends beyond the polygon I want to cut
c) if I create a new polyline in my polyline .shp file, I can use it to cut the polygon (but I need to use the pre-existing polylines for cutting) [I tried this after reading the bug report http://hub.qgis.org/issues/15312 ]
I had wanted to add this question as a comment to a previous Question on this tool but my StackExchange standing isn't high enough: Is there a way to split polygons by several lines in QGIS?