What have you got "Disable Overrides" in the "Extra Photo Operations" menu set to? If it's not set to "Disabled" try setting it to that. When you have overrides set they should appear in red on the screen - if it says "NO OVERRIDES" then overrides are disabled.
Also, if your camera has its own "Manual Focus" mode then that needs to be enabled before focus
overrides will work.
Otherwise, maybe if you are in full auto or easy mode try switching to program mode instead.
Does that help?