M.B.
Is she still wearing diapers or are you potty training her? Sometimes it's easier to get a stool out on the toilet than in a diaper because the big ones have more room to go somewhere. Stick her in a nice warm bath and let her play and relax. She might go in the tub which is gross, but if you watch closely enough you might be able to tell when it's coming and scoop her out and put her on the potty to go. I think the key is probably going to be to get her to relax and not think about it. Give her a book on the potty and sit and read it with her. Maybe plop her in front of the tv with a special video you never let her watch on her potty chair.