Slaughtering Chickens? Does anyone else have problems with the broomstick method?

A month ago we were slaughtering chickens at a friends house and we used the broomstick method where you put the broom over the neck step on both sides and pull up the legs and it breaks the neck. Well we did it on the first chicken and the head came off still attached by a throat we said ok and tried it again it happened again. What could be the problem? Is it just Us?