Except for Apollo 11 which has always been assumed to have been knocked over by the blast of take off.

Lunar Reconnaissance Orbiter Camera images of each Apollo site taken at roughly the same orientation but with different sun angles to show the travel of shadows. Combined with knowledge of the Apollo site maps which show where the flag was erected relative to the Lander, long shadows cast by the flags at three sites - Apollo 12, Apollo 16, and Apollo 17 - show that the these flags are still “flying”, held aloft by the poles. There is no indication of a flag shadow in this Apollo 11 image.
CREDIT: NASA