Judith's weaknesses are vanity, pride, and a lack of awareness.
Her vanity is evident when she sees Kit's fancy clothes. She becomes consumed by them until her father enters the room. When she tries on the peacock blue gown, she "tiptoe[s] across the door, straining to see herself in the one small dim mirror that [hangs] over a chest" (The Witch of Blackbird Pond, Chapter 4). Vanity is frowned upon in Puritan Wethersfield.
Judith's pride is evident throughout the novel. She does not like to admit when she is wrong. Judith sets her eyes on John Holbrook. Kit knows that John is drawn to Mercy. Judith is completely unaware of this fact. Kit notes what will happen if Judith finds out that John wants to court Mercy instead of her. She knows that Judith "is so proud. She'll put her nose in the air and pretend she never had such an idea [about marrying John] in her head" (Chapter 13).
Her lack of awareness is evident when she assumes John wants to pursue her romantically. John seeks to ask her father for permission to court Mercy. He goes to the house to speak to Matthew Wood and Judith assumes he is there to court her. She does not even let John ask his question. Instead, she addresses her father about the matter:
"Oh Father, you must have guessed. John doesn't need to tell you" (Chapter 13).
Judith's assumption forces John into a courtship he does not want. He does not wish to insult Judith or her family.