One will, one won't

[myfandtora]

I have two kittens, about 13 weeks old (Myf and Tora). I started them on the LK on the floor at 9 weeks (They were quite young when they came to me). We have since moved on to the amber with an insert (I used the lid of a takeaway container that I gaffed onto the amber well and truly), I've gradually been making the hole bigger, it's about 2cm x 4cm now.

Myf is fine. She's doing all her business in the LK, not aiming through the hole yet for 2s, but is for 1s. She will hold it, waiting for me to stand up, then bolting to the loo so that I can follow her and give her a treat. It's quite funny really, if I don't follow her, a few seconds later she emerges from the toilet and comes looking for me. She'll then weave and mew until I follow her into the loo.

Tora on the other hand was fine, she was getting it completely. All of a sudden she has gone off it. I come home every day to find a puddle next to the loo. She won't get up on the LK any more. She scratches around and squats in the loo, usually I intercept and place her on the LK. She'll then dig around but then hop off again. We have to repeat this a couple of times before she gives in and goes. (Except for poops. I place her on once and she goes right ahead.)

I try to make sure that the LK is as clean as possible.

Possible causes: She doesn't like Myf's scent on the LK?
Change of litter? (I changed brand, but not type - it's still just the crystal litter)
I've gone back to work after 6 weeks of holiday?

Help! I'm sick of puddles!