The lords of the Philistines came up to her, and said to her, Entice him, and see in which his great strength lies, and by what means we may prevail against him, that we may bind him to afflict him: and we will each give you of us eleven hundred [pieces] of silver.
The fact that Samson told Delilah to do something to his hair (Judges 16:13) suggests that he was giving her a clue to his strength. She did not pick this up but kept hounding him for his secret. Finally he gave in (Judges 16:17; cf. Judges 14:17).