Have you got the 'FN' lock enabled?
Somewhere on the keyboard is an Fn key (usually next to bottom left windows key), press that at the same time as F11 F12 and it should work
Also the Fn lock should be toggleable, on my inspiron 13 7000 its the escape key (Fn+Esc), which decides if the Fn lock is enabled, this changes whether your F11 key would lower brightness by default or 'F11' by default, with the Fn key being the mutator (like a shift key)
Hope that helps
