This problem has been already posted several times. -> Issue #0000408
http://feedback.dayzgame.com/view.php?id=408

Yesterday a developer postet a note on this site:
"Hello,

we are aware of this issue and are already working on a fix.

Please keep monitoring this issue for future updates on the situation.

Thank you for your feedback!"

-> http://dayz.gamepedia.com/Changelog#Known_Issues_2
"Ongoing melee balancing has zombies more resilient to body impacts. (Aim for the head)"

It's a known Bug. I hope it will be fixed soon.