Now she had liers-in-wait abiding in the inner chamber. She said to him, The Philistines are on you, Samson. He broke the cords, as a string of tow is broken when it touches the fire. So his strength was not known.
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).