Help, my cat is having problems using the box!

I have had my cat for almost three years now. Just about the entire time I have had her she has always had a hard time using the litter box to poop in. She urinates in it fine.I also have another cat too, with her own litter box, which she uses fine.

Back to the problem cat. I've tried different litter, places in the house, food etc..to no avail. What else can I do? Should I confine her to a limted space area and see what she does? Could this train her to use the litter box again?

Please help, i'm at my last straw now. I have a new born, and I just can't have my cat pooping randomly in the house. Thanks!