1) Place your own player head
2) Close Minecraft
3) Change your skin via minecraft.net
4) Run Minecraft
5) Your player head did not update, but your skin did
Placing player heads and/or giving more of your head does not fix the issue. The only way to fix is to place your head in a new world. This is also version-specific; updating the heads in 14w25b will not update heads in 1.7.9.