Here is a quick video of doing a Boolean diff on this part. The thing to note is my cutting surface I made punch all the way out both sides of the object.



I think the issue you are running into on this part is in the little half arc cup at the bottom piece. The little cup part is slightly out from the top shape which is doing the "little sliver doesn't work thing."

If you open the part without rotating it, it's in the back side. You can find it by doing an extrude strait down of the top profile the doing and isect curve on the 2 solids. This will reveal little fragments that will show you what is being described.