Is she kicking the calf off? Walking away?
We had a two year old many years ago that would not let her calf nurse. First time momma, and I am sure it hurt. We would put her in the chute and tail her while the calf nursed, but if the calf tried when we were not there, she would lift her back leg and hook it on the calf's neck and kick it away.
I got tired of the chute, and mad at the cow, so I bought a pair of hobbles and put them on the cow. She could walk still, but not pick her foot high enough to kick the calf off. Had the hobbles on for two days, and it fixed the problem. She stopped trying to kick. We removed the hobbles and she acted like she should. Never tried to kick off a calf again. She just needed to figure out what her job was, she was not mean and loved her baby but did not like the nursing.