QGIS Print Composer has numerous complex interacting features. Scale vs paper size vs map extent vs QGIS main window map extent - locked layers - resolution - map rendering level (i.e. the scale used to determine the map rendering), WMS tile zoom level choice, and so on. Things seem often not to work as expected.
Is there a source of full instructions explaining how these interact? The main documentation explains the basics fine, but not these complicated interactions.