5 week old kitten not using litter box

I got some new foster kittens a week ago, and I am having a litter issue with one of them that I've never had with other fosters -- though maybe I have just gotten lucky! 

They were "feral" when I got them a week ago at an estimated 4 weeks old, probably taken by the Humane Society from a feral mom. They warmed up to me quickly and are super cute. 

I've been keeping them in a big playpen type thing that has enough room for food, bedding and two small litter boxes, and I am using pellet litter. 

Two of them are great with going in the boxes, but one isn't -- at all. I've seen her poop in the box once, and then poop outside it, and I don't think she has peed in it once. Luckily I have the bottom of the pen lined with plastic because there is always a puddle or two when I wake up in the morning or come home from work. 

I worked from home today and they have been roaming around one room, but I have been putting the little offender into the litter box once every 30 minutes to try and teach her. I even added a third litter box outside the playpen. 

Well, I just put her in the litter box a few minutes ago. She jumped out of it, walked two feet into the little pen and peed on the plastic lining of the pen, about 10 inches from the litter box, as soon as I walked away. 

Any advice? Thanks!

Oh, and I am cleaning everything up with Nature's Miracle. If anyone can recommend something better that may turn her off from the spots she keeps peeing in, please advise!

Is she at least litterbox adjacent? That is to say, does she usually go just 10 inches from the box? Or does she go wherever?

Have you tried not just placing her in the box but showing her what to do (i.e. using her paw to scratch at the litter AND also using your own nails to do so?)

How small are the boxes? Is it possible she wants a bigger one even though she's a tiny tot herself? How clan do you keep them? It is possible you are keeping them too clean and she has nothing there to associate the smell with. Have you tried sprinkling a bit of cat attract litter on top of the pellets? Have you tried moving her deposits (both poo and urine) into the box after she goes elsewhere to get the smell in there?

5 weeks is still very young to be fully trained and it is not unusual that a kitten this age might still be failing to get it. Just continue to be patient and try to work with her.

I thought it was litter box adjacent, but I just discovered some on another plastic mat in the closet. At least she likes plastic?

The boxes are actually 9 x 13 aluminum pans, but maybe I should try something bigger. Or just wait and see since it might not be a size issue now that I found that pee elsewhere. 

I clean them once a day, really more for my own smelling sanity. All her litter mates are going in the box, you would think she would get it! 

I think I have some of the cat attract additive I will try, but if that doesn't work I will buy some kitten attract litter! 

Thanks!