We have shown a lot of heifers, and as you know they each have different personalities!
Some are leaders and some are followers. If she is a follower, try not to go into the ring in the first position. Sometimes they are picky about where you are in their line of vision, try slightly shifting your position a little forward or back and see how she reacts. Some also need a real light touch on the lead, they balk if you are forcing them. She may also react better with the show halter. They can really sense your frustration, so be patient! Good Luck!
When she stops...set her up. Keep working her, so she doesn't get the idea that she is in charge. She should get the hang of it.
I would probably try testing her with another animal in the lead position, and practice walking her in the same area each time, with the same pattern you would use at the show. If you are taking her on a "new route" each time she might just be nervous about her surroundings.