Sheffield valance with cording

A customer of mine wants a Sheffield valance with cording along the bottom edge. I've tried interfacing the fabric, and not interfacing the fabric..the face fabric will not lay entirely flat on the curves edge of the flat section. I've clipped it, it still appears puckered, although there are no puckers if that makes sense. How do I get the fabric to lay flat and tight? Anyone?

Yes I know the pattern suggests gluing any trim to the bottom edge. This is not an option. Need to fix the problem.

Thanks!