Hello,
1. Did you make the mesh so it is in top view going down on the screen from a Y=0 to a negative Y?
[Y -> is the axis going from a 0 around center of screen to either top (positive) or bottom (negative) part of the screen]
2. Did you make the mesh so it was centered around the X axis making the right side of the mesh about the same X value in positive number as the left side of the mesh is in negative value on the X axis?
[X -> is the axis going from a 0 around center of screen to either right side (positive) or left side (negative) part of the screen]
Those two are the most important ones in making a mesh act like a spline as a spline operates around the idea/principles that the center of it is equal a X axis value of 0 in a modeling program. And, it is drawn/dragged out along the negative value of the Y axis so all parts has to be along that path to show up in the game.
Hope this helps some?
Best of luck
Linda