Pooping outside their litter box

I have a 3 year old female rag doll. She's living with a 5 year old male ragdoll. Before, they were using paper based cat litter then we switched to a wood based one. After a few months, my mother in law came to live with us. Then, last month, she started pooping outside the litter box and in front of the sofa. We tried telling her off, changing back the litter to the old one, and using products to mask the odour. But nothing worked. How can we stop this behavior?

We live in the UK and we haven't contacted the vet. She and the other cat are both neutered.

Thank you