Retrain a cat to use the litterbox

We have a problem with at least one of our cats not using the litter box to
do "Number 2", she will urinate in the box but when it comes to the other
she does it in the floor around the box.

A little background info: We have four cats in a house with about 1500 sq.
ft. of living space. Petra is the violator here, she is about 10 years old
and weighs about 15 pounds. After we had Petra for about 5 years we adopted
Cleo and they lived together fine for about 2 or 3 years with no issues
(sharing litter box, food bowl, etc). Then a year or so after we moved into
this house we found a kitten in pretty bad shape, hiding in the bushes
outside our house and took her in to take care of her and eventually adopted
her as well. Petra and Cleo never really bonded with Connie (cat #3) and
this is when Petra started using the floor instead of the box. We have now
rescued another little kitten, Leela, however the issue hasn't really gotten
worse. We believe it to be that Petra doesn't want to share a box with other
cats (except for Cleo) so we've tried to get multiple boxes. This doesn't
work however because the other cats just use whatever box they want to use.
I believe it's gotten to the point now that Petra doesn't even check the
status of a box when she needs to go, she just goes on the floor.

We currently have two boxes downstairs and an automatic box upstairs. The
automatic box is almost too small for the "Portly Petra" to fit into but
we've seen her do it.

We are seriously considering having a baby in the next year or so, but I
would like to fix this problem before we do since it obviously lowers the
quality of the air in the house.

Anyone have any ideas? Any thing would be greatly appreciated!

-Matt