Constrain to face or Turn to face problems

Hi! I am having huge problems with both the turn to face and constrain to face methods. I am trying to make a zombie face my main character so that, no matter where the character goes, the zombie will always try to follow him...however, insted of facing the character and moving toward his direction, the zombie does this wierd thing where it shifts around for a second(as if it is confused) until it decides on some random direction to go that is no where near the direction of the character. What is the problem? Please help.