Hi,
I don't think that is the problem, you can start at any elevation but you need to start at same station. Snapp your first vertex to the left frame or extend it with a vertical help line to any elevation you please. When you are urged to select the frame , select the horizontal or vertical part of the frame (not the help line)
As an option You may also change value for max to extend the vertikal axis of the frame (when creating the fram)
/Lars