The person most convinced they cannot be manipulated may simply require a more flattering form of manipulation.

Believing yourself immune is the single most useful trait in a target. It removes the checks, since nobody verifies what they are certain of, and it tells the persuader which direction to come from: appeal to the independence, the intelligence, the fact that you see through what others fall for. Flattery aimed at your immunity is still flattery.

Assume you can be influenced, because you can. Ask what an argument is offering you emotionally, especially when it makes you feel astute. The question is not whether you are being persuaded but by what, and people who assume they cannot be are the ones who never look.