Zoe on the Pot

If you can catch her getting ready, she'll go on the toilet. She loves it and is always so proud of herself. She'll say, "I did it" and then wipe herself 800 times before say, "I done". For some strange reason, everytime she's at Grandma Ni's she goes in the toilet everytime. I don't know what I'm doing wrong, but she doesn't mention it or think about it at home, until it's too late????