Making One Object Retain Another

I have a ring in a staple. When I run the simulation the ring falls through the staple. I need to know how to make the ring collide with the staple and so as to be retained by the staple. The file can be downloaded here:
The ring and staple objects where appended from a file where the simulation is working. The ring falls against the staple and swings from it. I have not been able to find what is different. What is missing to that will make it work in this new file?

I have solved this. The “Steps Per Second” setting in Properties “Editor => Scene => Rigid Body World” was a value too low for the simulation to detect a collision. After setting “Steps Per Second” to 600 (was 60) and “Solver Iterations” to 100 (was 10) the ring was retained by the staple when it fell and it swung as it should.