Icons stuck on second display!

I have a laptop in a docking station, a 23" 1080p monitor, and a 28" 4K monitor. For months, when docked and the lid closed, the 23" monitor is primary (login screen, menu bar, icons, etc.) and the 28" secondary. A couple of weeks ago, after some update, the monitors got reversed, so the 28" became primary and the 23" secondary. I got that fixed, and the 23" is primary again... but my icons are trapped on the 28" monitor. I cannot drag them over to the 23". I tried cut-and-paste, but that screwed some of them up. I cannot for the life of me figure out how to bring my icons home!