In this video you can see, for some reason, not every part of this shape will subtract even tho the shape clearly is big enough.
If you rotate your view with 180degrees, you should see that the rectangle is not fully selected (the faces common with the ring face are not selected) - can you please check
Here is a more detailed video:
Thank you - so the issue is that the ring profile is not selected. I grabbed a screenshot of your video, which shows that the rectangle is not selected (note that it is not highlighted)
In order to get the subtract right just tap on those two faces as well and it will work
Seems an imperfect system. What is the point of creating a workplane that aligns with a surface only to ignore certain areas?
Yes - the behaviour is imperfect, but this is the best solution we could come up with (if we would auto-select those rectangle faces as well, it would lead to different problems)