Kitten Poop!

To my fellow feral caretakers, have a question:

I posted pics recently of my two lovely semi-feral kittens, Miss
Marmalade (Marmie) and Harley. They are doing great for the most part,
except for one little problem - Marmie is not going in the litter box.
She goes *around* the litter box, but not in it.

She went to the vet right before I got her. She did have worms, she's
been treated. I haven't noticed further worm bits in her stools, and
believe me, I've been getting reeeaaal good looks (and smells). Other
than that she was healthy. Her poops are still a bit runny, but she's
regular and pees regularly too, though once again, not in the box, as I
just discovered. She was peeing in a cat bed, that has been removed.

Her brother goes in there like a dream. I clean up each "deposit" very
thoroughly with a special cleaner. I've tried placing their litter box
over the offending sites, no go. I've tried containing them to the
bathroom for awhile for feeding and post-feeding elimination, no go. She
held it, then after I let her out into her area, she went and pooped.
She was doing this when they were in their big dog crate too for the
first few days. I thought maybe the space was too small. She was pooping
in their sleeping area, too. And she was doing it before I got her, her
rescuer noticed it too.

I'm thinking another vet trip is in order, but maybe this is a habit
left over from her original worm condition? And also keep them contained
in the bathroom, a "virgin" room for a few days. I hate to back track
with them, as they're making great progress getting more freedom in the
house, but this little problem will prevent Marmie from getting adopted,
and I'd really like them to get adopted together.

Sorry for the long post, but I wanted to give all the info that I can
think of off the top of my head.

K