Comment 4 for bug 771394

This behaviour seems to be caused by the absence of the Policy Kit agent. When I started '/usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1' (which apparently had died) manually the problem was solved. The store shouldn't fail silently when a Policy Kit agent is not present.
The absence of a Policy Kit agent also affects for example the Updated Manager and the Software Centre which will simply fail silently to install packages instead of explaining to the user that it can't install because there is no Policy Kit agent running.