Apologies for the long post but wanted to cover everything we have tried with this to see if there is any more advice you can give.
We have recently brought home two kittens. They have the same dad and were born in the same house but two weeks apart so they think they are litter mates they are 11 and 13 weeks . They play together and sleep together wash each other and generally get on very well. The older one uses the litter tray with no problem and that has been the case from day one. We discovered the younger one had been going under a chair in the house so we cleaned and blocked that off and now she goes near to the litter trays but not in them. We have started to see the signs and put her in the tray and she then wants to get out and go near it but not in it. We put her back in and, after a couple of times, she will go in the tray. We have praised her for going, given her treats and then cleaned and sprayed a solution for ridding the smell. We have not created any drama with it and not chastised her at all. We are at home with them at the moment but cannot supervise her all the time. When we go out they are put in an upstairs room with plenty of toys and beds etc and with an adjoining bathroom where the litter tray is. She had started going for a wee in the litter tray independently yesterday and did several poos after some persuasion. I left them for a couple of hours this morning and she had done everything outside the tray again.
We have tried all the methods we can think of to encourage her. We used the same litter at first and have tried changing this. We always scoop out poo and have cleaned the trays every couple of days. There are two trays downstairs near where their 'play area' is as well as upstairs, two are completely uncovered and one is tucked under a bench but with plenty of room. Our house is quiet with no other pets and no children and we don't think her aversion is caused by us. They came from a house with adult cats, dogs and kids so very busy. We did notice on our second visit (when we collected them) that the one litter tray we could see in the house was full and smelt bad we don't know if there were more but could a previous dirty and overused tray be the cause of her aversion and how do we try to help her with that? any advice will be very gratefully received.