In order for iGrafx to automatically and uniquely name a line coming out of a decision diamond shape, it must come out of a unique location on the shape that does not already have a line leaving that point. In other words, you must use different locations when drawing your line from the Decision to get to separate named paths. If you draw two or more lines from the same point, then they will all be labeled with the same decision case.
One method to get to separate locations is to draw the line out of the diamond where there is not currently a line leaving the diamond. The other method is to move an existing line off of the location you want to draw another line (click on the line, place your cursor over the square at the beginning of the line, and drag it to a new location on the shape), and then draw the new line from the location where you have cleared a spot.