Hiya!
I had a similar problem when I hadone of my bengal girls. She did the solids in the tray mostly, but not the urine! So in the spots where she went, I cleaned thoroughly with a gentle cleaning product, then used a repellant (you can get these at the pet shop to spray for kittens and puppies). The smell usually stops them going to that place. But you may need to apply several times, depending on if she has been there several times. Secondly, I loked at different cat litters, and did some research. Indoor cats, and particularly kittens can dislike the 'feel'
of a litter on their feet. It may be too hard so they wont use it
I use a white chalky one now, available from catsan. She really took to it, but it took a bit of persistency. We also used a small amount of repellant in the tray initially. It was an uphill struggle but she is fine now. Do you have only one cat?
I have two bengals who live indoors. We have two trays. But for one cat I think your one large tray is enough.
Good luck with it anyway,
Julie