Mass Edit Mode Not Working…

I would love to be able to delete spam comments using edit/comments/mass edit mode/invert checkbox selection/delete checked comments. But it doesn’t seem to work. I get the dialogue box which reads “You are about to delete these comments permanently ‘Cancel’ to stop, ‘OK’ to delete”; and I click OK, of course. The screen refeshes. But the same list of comments appear on the refreshed screen — they have not been deleted. Does anyone know why this isn’t working for me?