Difficulty with Region Difference

As a Grasshopper beginner, I'm having difficulty working with the Region Difference component. I believe my problems reside in my lack of understanding of data trees and what type of data the component is looking for.

I'm making a definition that will take a surface, divide it into contours and ribs, and lay out the lines for laser cutting. I created one that made solids and did a "solid difference", and it worked fine. I'm trying to create one now that doesn't use solids, to speed up everything.

I figured out how to get the notches cut on the contours, after much swearing and trying to figure out how to manipulate the trees. I'm sure there is a better way to implement it, but it's working now.

I'm trying to cut out the notches on the ribs, to be laser cut. Unfortunately, the method I used for the contours is not working on the ribs. Sometimes, it'll notch just the first part of the first 4 ribs, and then repeatedly notch the last one. Other times, it doesn't work at all.

What is the best way to structure the data to perform the region difference? Any idea of why this wouldn't be working?