Is the body fully seated on the motor? Is the camera or motor new to you, or has all this worked well for you in the past?
As you say, it's a fairly simple process. If it's not engaging the film spool, then it seems likely there is a gap between the motor and body, or something is blocking the upward travel of the shaft, or there there is some problem with the film spool that it can't reach the lug it's missing, etc.