Unable to install software using add/remove software

i am using the latest raspbian for robots, and have just found that when I am not using VNC but have my pi connected to a monitor I cannot install software using the preferences-> Add/Remove software. I click on a tick box, click ok, its says its installing (but never asks for a password), it completes really quickly, far quicker than expected. Then there is nothing installed… If I do the same via VNC then I do get a password request and all works as it should…

Could any one suggest what is happening. I am on a vanilla install, nothing is changed from the original image as downloaded.

That’s weird.
What if you do it from the terminal? Connect directly with monitor/keyboard, start a terminal window, and type
sudo apt-get update
If this one gives no error message, follow with
sudo apt-get upgrade

let’s see if those return any error code. (Those commands are what’s happening when you do a ‘Preferences -> Add/Remove software’)