I am using QGIS 3.16 to create new shapefiles from existing raster maps (p 225 in the 3.16 User Guide). My new polygons work OK when incorporated into other projects, but I cannot edit them to correct errors in their vertices. I am trying to use the protocol on page 485 in the 3.16 User Guide to (1) toggle editing, (2) left click an existing incorrect vertex once, (3) move the cursor to an improved position and (4) left click a second time, hoping to move the faulty vertex. Most of the time, I am able to select a vertex, but thereafter nothing happens at all, and the misplaced vertex remains in its original wrong position. On a few very rare occasions I have seen changes, perhaps after some unintended random change in my procedure.

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