A house divided - again

I made some changes to the pooping area in the spare bedroom.  I moved the smaller litter box to the laundry room where it's hardly ever used and moved the XL box into the bedroom.  I then located the box on the west wall instead of the south wall, providing a better line of sight to the door.  I'm grasping at straws here.  I moved another pet carrier to the original litter box location and various other boxes and stuff on the spots she's used before.  That was yesterday morning after I cleaned up the mess we found in the spare room.  Sassy has been having more 'accidents' than on purposes.  But she used the litter box last night.  That's the thing we can't wrap our brain around - she uses the litter box just fine some days and then other days she won't.  If we could find the trigger for either event, we would be happy to do that, say that, buy that, whatever it would take.  Sassy has taken up residence in my craft room again or on the couch in the family room.  Sadie is living almost exclusively in the basement, coming up for meals but not interacting in the family room like she used to.  Carl put a blanket down on top of a large bin he uses for storing photography props and she seems content to sleep there.  Carl spends a lot of time at his computer I think she likes Carl best, but that's ok, cuz Sassy loves me best.  So, from the freezing mid-west, I hope you all have a very warm and happy Thanksgiving.