Hi Andrew, yes please post a .3dm file with your curves in it instead of only a screenshot. That then makes it possible to do a detailed analysis of your objects and give you more accurate advise on what is going wrong.

There are a few different things that can make you only get "side walls" on an extrusion like that, it can happen if your curve isn't planar, if it isn't closed like if there is a gap between the starting and endpoint, or also possibly if it isn't well formed like if it has any areas where it is self intersecting. If you could post the .3dm file it would be possible to examine your object more closely to determine exactly what is going on.

3D modeling operations are more sensitive to defects such as self-intersection than 2D illustration use and it's not too unusual for curves done in a 2D illustration program to need some amount of cleanup work to improve their geometry before they can be used directly for 3D model construction.

- Michael