Potty training and public bathrooms?

My 20 month old is being potty training, she potties without an accident for weeks and she doesn't need a diaper during nap time but wears one during the night just in case. Well we have a problem when it comes to public bathrooms, she said she has to go potty when I take her she sits on the toilet but doesn't go at all. I've taken her to the bathroom twice last time we went to the restaurant but she still holds it in once we get home she goes right away. any of you guys have or had this issue? how did you solve it?