How can i litter train my 12wk old bunny rabbit?

i have a beautiful 12wk old bunny rabbit called Stew.she sleeps in her hutch and spends alot of the day and evening in the house with my daughter and i.she has a litter tray with straw in.but rarely uses it even though we put her in in lots.she has taken to pooh'ing on the sofa and she has started to wee there as well.how can i encourage her to use her litter tray? we put her pooh's in the tray when we collect them up off of the sofa! any advice will be gratefully received.thank you :)