I don’t know why this is not working.
If that’s showing the entire length of the Audio File then there won’t be any available Audio to slide into or out of view.
Try shortening the Audio Event by dragging the lower right corner to the left. Now can you slide the Audio around?
Slip editing is a little bit like scrolling a window.
If you have an Explorer or Finder window showing you the content of a folder and there are more entries than the window can show, you’ll get some scroll bars to move the visible area around.
An audio event is like such a window to an audio clip (audio file). If the entire content is display then you can’t move anything around.
thanx
thanx 
I ran into this same issue and it confounded me at first. I was used to slip editing in Ableton Live. No matter the boundaries in Live, even if you’re looking at the entire audio file, it allows you to slip edit. Great for adding nuance and not having to move the start and end points of the clip.