Jim Dandy: Some other factors and ideas...but no guaranteed solutions, I'm sorry to say.

I've been through this (well, not quite through yet) with one of my

Some ideas I've read (and some that I've tried):

Offering various kinds of litter.  Their textures vary a great deal, and some cats sure can be fussy, can't they?  Wow...box styles vary greatly, as well.  One of my girls can't stand those enclosed ones.  Another likes the enclosed style that's entered from the top!

Cleanliness of the litter box.  One vet insists upon this.  I usually only manage cleaning once per day; some insist on doing so even more frequently than that.

Number of boxes available.  One vet suggests a formula: have (n+1) boxes in the house, where n=the number of cats you have.

One of my girls seems to lie in wait in the middle of a narrow hallway that needed to be passed to get to the usual litter box.  Like some kind of troll one planning an ambush.  I think the girl with "the problem" was sometimes just steering clear of a showdown.

Large sheets of tinfoil placed on areas where the dirty deeds have tended to happen.  (I placed these across half of my bed for a couple of months.  My newest  was peeing there!  At least she's gotten over that problem.  So my bedroom's foil bedspread won't be featured in House Beautiful; no big deal.)

If you can be stealthy and patient, have some idea when it happens, and have good aim with a water pistol, catch your little darling in the act and shoot from a bit of a distance.  I think the idea is to get him/her to associate the unpleasant spray with their activity--but not associate it with YOU being angry.  Don't be yelling...or anything worse...when this takes place.  You've already got one problem to deal with.