Separating eggs without breaking the yolk isn't one of those problems that keeps me awake at night. Nevertheless, there are occasions where I can't get a single damn yolk to hold together and other times when I can do anything short of play a round of tennis with 'em.
Could the freshness of the egg determine how likely the yolk is to break? Or maybe the temperature of the egg?
Generally the problem comes not when I crack the egg, but when I start to transfer the yolk from shell to shell.