Hi Mark, actually the Trim command is cutting the surfaces of the object but it is having some difficulty figuring out how to group the different surfaces together into different pickable fragments.

So for example in the model you posted, if you select this plane here:



And then just delete it to get rid of it, you'll see that the slanted block has actually a cut mark on it from some previous run of Trim on that block, it looks like this:



So now you can select some of the faces that you don't want. You can select a face sub-object similar to how you select an edge, by clicking a second time on a solid to "drill in" - but for selecting faces you may need to zoom in a bit so that you can find a good spot that is not nearby an edge to click on because edges get priority. So selecting those faces would look like this:



Then delete those:




There is a bug in Trim but at least it did actually put in dividing edges so that you can remove stuff manually.

I'll investigate to see if I can figure out why Trim is not able to figure out the different groupings of the cut up pieces properly. Thanks for posting the geometry so I can examine it.

- Michael