I've installed 16.04 on my Asus UX21 laptop. I installed Unity Tweak Tool and added the snwh/pulp repository (from Noobslab website) so I could install the Paper theme and icon set.
Sometimes when I log in, the Ambiance theme is active and Unity Tweak Tool is unresponsive when trying to revert back to the Paper theme. A reboot usually solves the problem.
Any ideas as to why this is happening?