I have this simple harness in SolidWorks 2014:

When I try to flatten it, I get this error alert:

This Route Assembly cannot be flattened as it contains loops and/or disjointed route segments.

So, I went to edit the route and edit the wires to see what is going on:

When I click on Select Path, I see this:

and this

Is that the problem that this wire is made up of more than a single segment? I had to do that to coax the wire around others, specifically by inserting a spline segment. There is complete tangency between segments, and the end points of the three segment are coincident, where the segments join.

How do I fix this?