I have the same litter box and the same problem, I love the litter box more than any other litter box I have tried and with a cat that is having litter box issues and 23 foster so far this year, I have tried a lot. The clumps sticking to the bottom is a problem though and it did not seem to matter what type of litter I used. I used a trial size of a non-stick litter box spray and it seemed to work ok. Of course I could not find it again after that, but I did find something similar to it on petco's website. I have not tried this particular product yet, but maybe it will help. If it works for you, feel free to let me know.
The Best Cat Litter
3 Ways to Clean a Litter Box - wikiHow
The present invention relates to a cat litter sifting device for use with litter which clumps when wetted, allowing for physical removal of the wetted litter along with fecal matter. Cats can be trained to urinate and defecate in a specially provided litter box filled with absorbent material. The urine soaked litter can be separated from the clean litter with the fecal matter if the litter clumps when it is wetted into agglomerates that are sufficiently large and stable to be removed like feces. Litters that agglomerate when they are wetted are called clumping or scoopable litters. If the litter is not of the clumping kind, in a few days, the urine odor will become so strong that the entire contents of the litter box must be discarded. On the other hand if the litter clumps so that the urine balls can be removed with the solid waste, the remaining litter need not be discarded.
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Simply select Autoship at checkout for easy regular deliveries. Petmate's disposable plastic liners make cleanup a snap. Spread liner over litter pan so that it overhangs rim on all sides. Sift out solid waste daily. This product is not a toy.
Last Updated: March 7, References Approved. This article was co-authored by our trained team of editors and researchers who validated it for accuracy and comprehensiveness. There are 16 references cited in this article, which can be found at the bottom of the page.