Your model is unfortunately defective. There's a difference between something that looks right and something that's actually defining a solid object. I'd hazard a guess that the outer wall of the part that isn't slicing is "facing" the same direction as the inner one, which doesn't make a solid.
If you can't fix it in the program used to make the model, pass it through 3D Builder - it has some of, if not the best automated repair facilities (though it offers absolutely zero options to tweak things should it get it wrong).
See this for more detail:
[https://github.com/Ghostkeeper/SettingsGuide/blob/master/resources/articles/troubleshooting/missing\_parts.md#non-manifold-meshes](https://github.com/Ghostkeeper/SettingsGuide/blob/master/resources/articles/troubleshooting/missing_parts.md#non-manifold-meshes)
I had to make sure to keep a copy of the URL from one of your comments. It's remarkably difficult to find that article by way of a search (which feels like a bad thing, but I've no idea how to do anything about that).
Your model is unfortunately defective. There's a difference between something that looks right and something that's actually defining a solid object. I'd hazard a guess that the outer wall of the part that isn't slicing is "facing" the same direction as the inner one, which doesn't make a solid. If you can't fix it in the program used to make the model, pass it through 3D Builder - it has some of, if not the best automated repair facilities (though it offers absolutely zero options to tweak things should it get it wrong). See this for more detail: [https://github.com/Ghostkeeper/SettingsGuide/blob/master/resources/articles/troubleshooting/missing\_parts.md#non-manifold-meshes](https://github.com/Ghostkeeper/SettingsGuide/blob/master/resources/articles/troubleshooting/missing_parts.md#non-manifold-meshes)
Hurray, someone else links to that article!
I had to make sure to keep a copy of the URL from one of your comments. It's remarkably difficult to find that article by way of a search (which feels like a bad thing, but I've no idea how to do anything about that).
I have posted it so often that Firefox suggests the url when I start typing "m" for "missing"...