4 year old won't go potty at daycare

My daughter has been potty trained for mom and dad for awhile but refuses to go potty at daycare. Only wears underwear at home and when we are out. No issues for Mom and Dad. She will sit on the potty at school but doesn't go so she is stuck wearing pullups. It's been this way for over a year. She's been in the same room. They won't let her go to Pre-K. I do think her previous teacher was not so patient with her. She's very private about going to the bathroom. I've spoken to her pediatrician and tried some of her suggestions but nothing seems to work. It's behavior and I don't know how to help her