Sweep1 along a polyline

Hello everyone,

I'm facing a small problem.

I would like to make a sweep along a polyline using a starting section (two red lines, bottom left hand corner) and an ending section (two red lines, top right hand corner).

I've just connected both sections to the sweep component.

In the example above, there's no apparent problem.

But in some cases, I come up against this issue :

The surface is divided into several parts no matter if the starting and ending section are different or not. 

For both examples (success and failure), I am using the same kind of data (3D polyline for the rail and two sets of lines for the section).

Could you help me to find out what could be the problem ?

Thank you ! 

Radis