Try and monitor exactly when she's having these accidents and then look at what she's doing at the time, maybe she's just too busy to realize she needs the toilet until it's too late. Use reward charts to encourage her to use the toiilet appropriatly and try not to blame her when she does have an accident, the less reaction she gets from the poo pants and the more praise she gets for going to the toilet should encourage her to turn her behaviour around, be patient, but if this continues for any lengh of time speak with your health visitor or G.P about it as occasionally there can be medical reasons why this is happening.