Judges 16:9

‘ Now she had liers-in-wait waiting in her inner chamber. And she said to him, “The Philistines are upon you, Samson.” And he broke the withes as a string of tow is broken when it touches the fire. So his strength was not known.'

It is clear that Delilah played it as a game. The Philistines dared not enter until they were sure the method would work, and thus Samson never knew of their presence, but rather Delilah cried out that the Philistines were there, to see his reaction and the result. This first time he may well have thought she meant it so he broke the withes but found no one there. And she no doubt laughed as though it were a game. But underneath her heart was beating rapidly and she was afraid. And she knew that he had not told her his secret, and that she did not know in what his strength lay.