Problem with use of litter box. Help!

We got two adorable kittens last Nov and they
took to the litter box w/o any issue. That's a total
of four house cats and one that goes outdoors too.

We've come to realize that one of these kittens
(now pretty close to fully grown) is not consistantly using
the litter box. She'll squat and pee (or poo) in front of us
and in rooms with boxes. We now have 7 litter boxes
that we scoop 2X a day.

We went thru the house with a black light and saturated
all spots with the enzymes to try to eliminate the odor.
Plus I replaced all the small rugs (or pulled them up).
No change - in fact she poo'd & pee'd on one of the
new bathmats w/in hours of me putting it down.

Sooooo... we're going to try the pheromones but are not
hopeful that will work.

Any ideas?