Potty training a one year old dog??

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My brother has a 1 year old German Shepherd/Lab mix she is not potty trained yet despite my efforts to help my brother. Frequent outdoor trips to potty, plenty of praise and reward after going outside, correcting when she goes indoors, long walks outside, etc. My Golden Retriever was housebroken within a few days she was only 9 weeks old. His dog becomes very excited by being outside and forgets I guess to potty outside because immediately upon entering the house she goes potty. I've never had a dog so hard to housebreak, she is a spayed female ontop of it. Suggestions???