Kitten wont use litter tray

[Lilly15]

I got a kitten about 2 weeks ago shes about 8/9 weeks old, she uses her litter tray sometimes, but most of the time she poos near the front door, under my bed anywhere really even though there's a litter tray in front of her. She also pees in her litter tray fine. I thought at first it might have been because she was nervous, but shes settled in fine and shes a really nice kitten. How can I train her use the litter tray all the time? Or any reason she wont use it? Thanks in advance!