Litter box cattitude

Question
We have 4 cats (2 adults, 2 half-grown kittens).  In accordance with the wisdom to have at LEAST 1 litter box for every 2 cats, we have 2 litterboxes.  However, one of them is consistently OVER-used, and the other one almost ignored.  They are located one in each of two bathrooms, in the only available spaces where they could be (or fit!).  I draw the line at litter boxes in the bedroom, kitchen or living/dining room! ;-)

Any ideas?  Are they just too lazy to waltz down the hall?  Or are they just being twits?

Our eldest cat (age 6) has epilepsy, and is 'stoned' most of the time on phenobarbitol.  She has the worst problem...often using the hallway rug...(deliberately!)...or else, getting in the litter box with her 'business end' hanging off the edge......you get the picture.  With her, I blame the drugs...but the others don't have that excuse.  

It just seems that the opposite of the "kitty care instructions" is true...that rather than needing a litter box each, or 1 per 2, they would rather 'compete' over the one!  I am puzzled.
Any ideas?

Answer
With the one that is using the floor, I would clean the area with an odor cleaner and also I would put tin foil where it does it most often. Tin foil most cats hate the noise if I recall.

As for why they don't use the other box, maybe it's not clean enough. Cats are very picky. Or maybe they don't know of it. Show it to them every day for a week and see if this makes a difference. Clean your litter box often and make sure it's done well because cats are picky with litter boxes. Good luck and sorry

Talk to a vet for help.