Why Can't I Restore The Preview Pane?

I am using Mail in Classic mode. I opened up the message pane all the way to review a list of messages, and now I can not restore the preview pane. Yes, there is a dot at the bottom of the message pane that shows an up arrow when I hover over it. But nothing happens when I try to drag the bar up. How do I restore the preview pane so I have an open pane below the message pane?