Looking over this, what is happening is that Functions from a Blueprint Interface with no output are considered Events by the Blueprints that implement that Interface. Those won’t show up in the Functions section of the My Blueprints tab. What you would need to do to override their functionality is search for the Event version of Function from the right-click context menu and add that Event to your Event Graph.
Our documentation page on this doesn’t reflect this information currently, but we are working to have it updated.
Here’s the documentation page, if needed: https://docs.unrealengine.com/latest/INT/Engine/Blueprints/UserGuide/Types/Interface/UsingInterfaces/index.html
Hope that helps!
-Steve