Dog Sometimes Poops Inside?

I have a 12 month old Cocker Spaniel cross. She was rescued from a puppy farm and while she is lovely, she has been hard work to house train. On the whole she is good and we keep our back door open to give her easy access to the garden where she will pee and poo, though occasionally she will poop in our sitting room on our rug. We are at a loss to understand how to fix this and any suggestions would be really appreciated.