We have 3 cats, they have 2 boxes. The boxes get cleaned every two days. One cat will urinate but not deficate in the box. This happens intermittently - it doesn't have to do with new litter or anything like that. She also is the least friendly of the three and will leave the area where the three are eating if one of the other two are pushy. All of the cats are over 1 year old and have been together for almost 10 months now. What to do???