Why are the network resources empty in Safari timelines, even if resources are there?

Every single page I go to, the Timelines->NetworkRequests tab shows lots of requests and their timing (top half of the pane), but the bottom half is empty, making it impossible for me to see what requests were made.

I know this is something simple I am overlooking, but for the life of me, I cannot figure out what it is.

Here is an image from loading Wikipedia: