litter box duties

i must say that i am concerned about stephanie. she realy seems to be letting herself go. first i noticed she was coming home sometimes and napping in the middle of the day. and i am pretty sure she is packing on the pounds. here is a pic i found of her from last month and one from this week. it may be hard to tell from the pics, but she definately feels squishier to me when i lay on her for my naps. but the worst part is she hasn't taken care of my litter box in months! it seems richie has now taken over this responsibilty, but he does not do nearly as good a job as steph. what has gotten into her? i am trying to decide if extra cuddling or ankle biting is the best approach to get her back to her litter box duties.