CAT MUMS! Help! Why won't she give up the litter tray!!!

[StinkEye]

We got 2 kittens last October, introduced them to the outside world about 4 months ago. One of the kittens totally got it, and now uses the garden as her toilet (we have a cat flap). The other one refuses. She will go outside, spend plenty of time out there, and come back in and use the tray. We're starting to put the tray outside and she will use the tray when it's out there, but what with the rain, it's been hard going.

Any advice at all to help get rid? The stench of her poos (she doesn't bury them!) even when they are immediately removed is doing us in!
Am i missing a really simple trick?

Well ok no it’s not but I wouldn’t want to go outside to toilet in bad weather. What do they do over night?.

Maybe the kitten is feeling insecure and wants to mark its place in the home? My girl has just started using the back garden and she's 9, her older brother died in March and he went outside but she always came in and used the tray. Not sure what the change is about, either she doesn't need to make herself 'known' in the house now or if she's marking the garden to keep other cats away. I found Royal Canin babycat and kitten biscuits helped reduced the smell when she was younger.

Dcat will only go indoors and if she's had a wee in one tray, she'll use the other clean one next time. She won't use the same one twice and will stand next to it shouting until someone refreshes it, then she'll use it again. She's free to roam in the day and in at night, but she'll be out and about, come in to do a massive poo, then trot out again. I don't think there's much we can do and to be honest, at least she's not going all over the gardens.

@StinkEye It's the Ferplast outdoor litter box. Works really well for us.

https://www.zooplus.com/shop/cats/catlitterrlitterboxes/outdoorrcat_trays/592372