Trouble Shooting Tips
Problem Cause and Resolution
Map axis handle will not lock to a specific vertex. More than three vertices are currently selected. Deselect all and only select the three vertices corresponding to the T/L, T/R, and B/L handles.

3D Cursor is not directly indicating a vertex. Place the 3D Cursor near the desired vertex lock point and press the F2 key to snap to the desired lock vertex.