I replaced the head for a G2 character with a human skull for animation. The head comes detached during animation. The neck protrudes out of the head when the character is dancing. Below, please look at the gif below. This has happened to me far too often that I am not able to replace the head successfully and still apply animations. I would love to know how to fix this problem. Thanks everyone for your help.

P.S. In the pic below, look at the neck of the male character on the right. His head is not fitted correctly and its going all over. Same happens to the female character on the left side (not shown in this pic). Head sometimes gets detached when animation is applied.