You are the only one who can open that particular link, so I can't see what you did or missed on that puzzle.
All the puzzles I have done are possible to complete. You need to work each clue and mark all the negatives in order to determine the positives - i.e. Mark finished some time before the Dodge owner; Mark is not last, the Dodge owner is not first (or anyplace after Mark) Mark does not own the Dodge.
Keep working at them and enjoy yourself, I laugh at myself when I make an obvious mistake, then make it again the next time! So no, I don't think you're thick, consider that you're human and our brains don't always work like computers!