won't go to toilet here, will at Nana's?!

Hi all,
as stated, my 2 1/2 year old dd won't go to the toilet here with us, but will do so proudly for her nan at their house - she'll even ask to go!
Then at home, even if I know she's about to do something in her nappy and ask if she wants to sit on the toilet, she'll refuse.
What am I doing wrong?
I always figured I'd wait til she was ready and asked to go, but if she will do it there...?
The best she's done with me is to ask to go (generally when I'm breastfeeding her baby brother or otherwise equally occupied) and then 30 seconds after I drop everything and pop her on the toilet she wants to hop off.
Any thoughts would be appreciated.
it's rediculous how personally I'm taking this :rolleyes:

Totally normal from my end! I had much the same thing with DS. He always went perfectly while out but at home he was too lazy and uninterested in going to the toilet. I put it down to being a novelty while they are out. Also with the 30 second thing my DD did the same and I never knew if I should let it go or try and make her stay longer. Depending on how long it had been since she last went I either let it go or sang songs to encorage her to sit longer. In the end she got the hang if it :) It WILL happen! :lol: I figured I'd rather her get on the loo thousands of times than clean up wee all day long. At least it shows she knows what to do, kinda keep up the practice for the main event, lol. HTH