‘ And she fastened it with the pin, and said to him, “The Philistines are upon you, Samson.” And he awoke from his sleep and plucked away the pin of the beam and the warp threads.'
While he was asleep Delilah carried out the process he had described, finally fastening his hair to the beam with the pin. Then she gave him warning of the presence of Philistines and he woke up and freed his hair, possibly breaking the loom in the process. He thought it was all part of the continuing game. He did not dream that previously Philistines had actually been present, just in case it worked.
Ominously ‘fastened the pin' are the same words as ‘drove the nail' in Judges 4:21. The blows were just as deadly for both had the same purpose in mind, the destruction of a man.
LXX has here, ‘and it came about that when he slept, Delilah took seven locks of his head, and wove them in the warp threads, and fastened them with a pin to the beam." Note that even in LXX there is no mention this time of Philistines actually being present. Perhaps this time she had decided to try to find his secret without the Philistines being present. It would be getting somewhat difficult having to explain to them each time why she had failed. She could then pass on the secret later and arrange to do it another time. That is certainly what she did in the end.