U3824DW, mouse and keyboard only work with one computer

New (huge) monitor is great.  I have hooked up the desktop to it iand it works absolutely fine, but when the HP laptop is connected for some reason the wireless mouse and keyboard (Logitech) don't work.  The receiver is plugged into the back of the monitor in one of the normal USB A ports and the image is coming through fine from the laptop.  I think it did work a couple of weeks a go but no longer.  Any help gratefully received!