How are you trying to get her to lead? One of the biggest mistakes I see people make is to start to play tug of war with them. Even as calves they are bigger and stronger than most people and you just wont win that fight. Are you doing this alone or do you have help? First and foremost before antyhing will learn to lead it has to trust you, so make sure you arent doing anything inadvertantly to prevent it from trusting you. Secondly, they have to have an incentive to move, for some of them our desire is simply not enough incentive to convince them. Generally the incentive is that when they take a step and come towards you it releases the pressure on their head. If she is sitting back and grinding her teeth then she probably isnt ever getting the opportunity to get that relief. If you have help try having them push her a little to get her to move up and relieve the pressure. I also find that sometimes, short gentle tugs on the halter get more than just sitting back against it and trying to drag them. Finally, take baby steps. When she takes one small step stop and reward her, scratch her, let her have some reliefe from the pressure on her head. Repeat that over and over, when she gets to where she takes that first step without hesitation, then ask for 2, slowly increase how far you are asking her to walk.
If all of that fails I have also had some that needed more reward...lol...once they were broke to tie good, I left them tied most of the time. They drank when I led them to water, ate when I led them to feed, etc...this is time consuming because you have to have the time work with them several times a day. But it worked pretty well on some of the really stubborn ones.
