Psychologists understand that people may not say what's on their minds either because they are unwilling or because they are unable to do so. For example, if asked "How much do you smoke?" a smoker who smokes 4 packs a day may purposely report smoking only 2 packs a day because they are embarrassed to admit the correct number. Or, the smoker may simply not answer the question, regarding it as a private matter. (These are examples of being unwilling to report a known answer.) But it is also possible that a smoker who smokes 4 packs a day may report smoking only 2 packs because they honestly believe they only smoke about 2 packs a day. (Unknowingly giving an incorrect answer is sometimes called self-deception; this illustrates being unable to give the desired answer).
The unwilling-unable distinction is like the difference between purposely hiding something from others and unconsciously hiding something from yourself.