Litter Box Problem..

I live in a house with my two kittens, one is about 7 months (Arthur) and the other is about 5 months (Sophia) and I had no problems at all training them to use there litter box.

So my problem is this: About a month ago Sophia started leaving "presents" (#2) in the bathtub. ugh.. I have tried changing litter brands and even tried moving the box to different areas of the house (right now it is in the bathroom). I take care of the box and keep it clean and free of waste.

I am not sure why all of the sudden she stopped using the box to go #2. any clues?

I would add a second litter box.
Cats often if given the chance use one for #1 and one for #2.
I would try that first.