Creating a through slot in object and can't repeat original success

Here is a quick video describing my dilemma. Thanks for the help

Looks like a version of the same issue with components we were looking at earlier. Your slot has a bounding box which indicates it is a component or group. As such it will not interact with anything outside it’s “wrapper”. You will need to right click and explode the component if you want it to interact with other geometry. There should not be a need to make the geometry a component.

Just use: Tools - Solid Tools - Subtract.
Make sure each object is its own group.