The phrase "upon you" means that they are ready to capture him. Alternate translation: "The Philistines are here to capture you" (See: [[rc://en/ta/man/translate/figs-idiom]])
It is implied that if Yahweh left Samson, he would no longer be strong. Alternate translation: "But he did not know that Yahweh had left him and that he would not be strong enough to defeat the Philistines" (See: [[rc://en/ta/man/translate/figs-explicit]])