My drivers side headlight won't switch from high beam to low beam.

Its either a bad bulb or a loose connection.

The high beam low beam thing is actually a mechanical (not electronic) shield that opens and closes like a little door...that's why it changes when you knock on it. Could be a stuck spring or jammed part. BTW...my service center had no idea it worked this way. When they couldn't figure it out, I took the whole thing apart and was amazed to find a simple mechanical device on such a hi-tech car.