I've got the problem...can't figure out how to stop Kiki from pooping outside the box.

2 years ago we adopted two babies and added then to our three kitten home. All four of then get along except for one little girl that does not hang out with the pack.  I think she is afraid that their playing is fighting. I feel horrible for her. Well so now she is pooping in weird places. On the bed, on the kitchen chair. I get she is trying to tell me that she is not happy but i don't know how to fix a problem like this.

How many cats do you have and how many boxes do you have?  Do you run Feliway diffusers or use any calming agents like Rescue Remedy or Spirit Essences?  My kitty used to poop inappropriately when she was stressed out.

We have 5 kitties and 6 litter boxes. They were all using the system well. I forgot I do have a calmer for their food. I'll give that a try. I do think it's anxiety. Thank you very much for responding to me. Here is Kiki baby.