I have the same problem with both headrests in the back seat of my week old 128i convertible. When I took delivery of the car, I noticed the headrests in the back had about 2" of height to them, exposing their chrome support rods. I'm only 5'-8", so when I look in the rear view mirror, the rear headrests at this height obscure my view. Being able to lower them all the way could help give me a little more rearward visibility.
I'll try the screwdriver/pick method mentioned on this thread to see it I can get it to drop.
2013 X3 28i ///M Sport | 2012 128i Cabrio | 2009 Yamaha R6 | 2010 Mercedes C300 Sport (retired) | 2006 MINI Cooper S (retired)
Occasionally I write about new cars over at TFLcar.com