My 5 Year Old Granddaughter Poops In Her Pants. She

My 5 year old granddaughter poops in her pants. She knows when she has to go. When she's asked why she doesn't go in the potty she says "I don't know." . She will run in the bathroom and urinate and come out and poop her pants. I think she tries to hold it in. Last year she would try to hold it and then when she would poop it would be huge and clog up the toilet. I know it was painful for her. I've told her that it's easier to go if you don't try to hold it in. She was doing really good for a couple of months, then relapsed. We don't know what to do. We have tried explaining to her, prizes for going, punishment, her parents have spanked her. Nothing is working. My husband thinks her parents have failed her as parents. I do not agree.