In the Season 8 episode Flatline, George, a graffiti cleaner, was shown the psychic paper by Clara, being told that she worked for Health and Safety. George then said that the paper was blank. The only other time I can recall that this has happened was during Season 2 when a Torchwood scientist said it was blank because the scientist had "psychic training." The Doctor said George must have been "unimaginative," but Clara even said she was from Health and Safety before showing him the paper, removing the need for imagination. George obviously didn't have psychic training, so how did he not see anything in the paper?
Despite appearances to the contrary, psychic paper does not always work.
It can be defeated by:
Strength of mind: Shakespeare and genius intellects in general are immune to it.
Psychic training: Torchwood employees were given training to resist it.
Psychic resistance: telepathic minds are proof against its invasive properties.
Sheer lack of imagination: George in the episode Flatline.
When psychic paper is in play, people see what they expect to see, usually an authority they respect and expected to see on the scene. George respected no authorities, thus the paper would have NO meaning and no means by which to interact with him at all.
That was answered in the episode itself. Immediately after George said the paper was blank.
The Doctor said it takes a complete lack of imagination to see nothing on the paper.
The Doctor hinted at this with his comment about George's lack of imagination, but I interpreted it further as a not-so-veiled reference to the way George behaved in general throughout the episode: prejudiced, bigoted and narrow minded. It was a social statement reminding us to remain open minded when dealing with others, rather than judging them as being "beneath" us. It was a nice counterpoint to the Doctor's own remarks about Riggsy's assumed lack of intelligence, wrapped in a bow at the end of the episode when the Doctor pointedly reminded Clara that being an excellent Doctor is not the same as being good.