Drag-and-drop background transfer doesn't start automatically

When I drag a file from Windows Explorer into a folder on my server, the background queue appears on the bottom but the new transfer item just says "Waiting...". In order to begin the transfer, I have to double-click it or right-click and select "Execute Now". There are no other transfers active and my background queue is set to transfer 2 items simultaneously.

Have I accidentally changed a setting that causes it to wait before executing a drag-and-drop transfer?

You probably have Enable queue processing by default turned off:
https://winscp.net/eng/docs/ui_pref_background

That did it, thanks. I thought I had tried that option, but maybe it only takes effect with a new connection, or I messed up something else.