My 7 week old baby is having a hard time latching on. Help?

Sometimes she latches right on and then after a minute or two she pulls off and then she can latch back on and gets mad. I have to put a pacifier in her mouth and then yank it out and stick her to my and then she latches on again for a bit and then I have to keep repeating. She is gain weight fine so she is getting enough milk and she knows how to latch on because she will did it right away but she won't stay on. I have tried to take the pacifier away from her but she gets frustrated and won't latch on without it anymore. Any ideas on how to get her to stay latched on.