Drip Rail Won't Snap On

[JC60B]

I'm in the middle of installing the Drip Rails on my '29 Briggs Fordor Leatherback. The inner portions are nailed onto the car. The outers are cut to length, custom bent to the contour of the car, and painted. Problem is I cannot seem to snap the outers onto the inners. I tried a smaller section out of the part cut off and that will snap on. It's just that I can't snap the whole length on. Anyone have any tips on snapping this in place?
Thanks