More surface trimming

The purple surface and the blue surface clearly intersect, and intersect fully enough to do a mutual trim. But it simply won’t work. Cases like this normally are caused by errors on one of the bodies, but I have thoroughly checked both bodies with Check, and VOR. I can’t find anything that would indicate that anything is even remotely wrong or questionable with either body. One body is a simple extrude. The other is a compound surface body. It is somewhat complicated by the fact that there are some interior faces which also need to be trimmed, but everything is clean.

The error is that when I select the Selections To Keep box, nothing highlights. The need for this edit came because the Trim Tools selection lost a selection. I know there has to be a reason for this, but I don’t know what I need to know in order to troubleshoot this. In the end, I got around it with a work around using a new feature rather than editing an existing one, and selecting the features in a different order. This is what I call the “random button pushing method” because I don’t have any rational reason for trying that. There is no way I would have predicted that that would work when the other method didn’t.

If you wonder why I get so wound up about stuff, just imagine you woke up one day and your cut extrude or revolve features wouldn’t work. How would you react?