Hi cad-guy, it would help if you could post the .3dm model file instead of only a screenshot.

But best guess looking at your screenshot is you probably have a little opening in the curve in this area here, I notice the vertical edge is a little darker there that's because there are 2 edges there not just one:



So in the top view go zoom in to that spot to look for an opening, that will need to get sealed up. If you need help with it please post the .3dm file.

- Michael