Issue: cat won't hop on the toilet

[Sergio]

Hello, I have a couple of young cats, Stella (2 yo) and Neo (9 months), both neutered.

We started with the Litter Kwitter some months ago, and it all went perfect until we started with the orange stage. Stella started pooping and peeing on the floor.

We switched back to the red Stage: It all went back to normal, both cats used the LK on top of the toilet until one day, when Stella started peeing and pooping on the floor again, so we decided to put the LK on the floor, with the red disk. They've been using it for some time with no problems.

We decided to buy the Multikatkit and start again placing the LK on top of the toilet, but the problem is that Stella is unable/unwilling to use it! The younger cat, Neo, is able to use it with no issues whatsoever, but Stella only likes it if I put it down on the floor again. How can I change her behaviour? I've tried putting her on the LK, but meows and flees, tried with treats too, but it isn't working.
Any advice?