Hoping for some advice! I have 2 cats from the same mother. Boy cat is just over a year old and never had an issue using the litter tray, then we got his sister who is now 7 months old and she struggles. When we first brought her home in Sept she was pooing anywhere and everywhere. I then discovered she had been using the cellar as a toilet and the stench was horrendous so we stopped them going down there.
Now she will use the litter tray but she is very very particular. She only likes fresh clean litter (which is costing a small fortune), and she will only use it when it is pushed up against the back door - If we have to move it to the side to use the backdoor, she will urinate on the floor (we have puppy pads down for this reason) and won't use the tray until it is put back in the position in front of the door. We have tried treats when she uses the tray and positive praise to her.