Apple ID password does not work

So, I'm trying to set up my mother's Apple ID and of course, it's already set up. The problem is everytime we enter the password, it says it's incorrect and that password is right to us. I use the same email provider as her and I get in but she doesn't. She's had this problem, with another Apple ID since my father apparently keeps changing the password, that's why I got her a new one. Can ANYONE help me?