I think to many people try to get in a tug of war with the calf, when the calf sulls up to man people try to just lean into the rope and this never works, remember when you get into the tug of war your calf will always win, they are bigger and stronger than you. The other thing is people constantly keep pressure on the halter even when the calf is moving, this will make them start sitting back on you as well. Remember that a halter is supposed to work by teaching them to give to the pressure on the halter, and when they give to that pressure then the pressure should be relieved, however when they move towards the pressure hoping for relief and you constantly are tugging then they eventually just give up on trying to get relief that way and learn to sit back and fight it. Apply gentle pressure, slowly increasing it until you get the desired response. Even if the calf only takes one step as soon as they take that step release the pressure on the halter. When they are giving to the pressure fairly quickly ask them for two steps, keep working up until they learn that there will not be any pressure if they just go with you.