This could be due to an issue with your parking pawl that is located inside your transmission. When you park on a hill and put your car into park your car will rock back or forward slightly and then it rests on the parking pawl. Over time this can damage your parking pawl and it will make it difficult to get your transmission out of park and into gear. When you park on an incline of any type while still in gear set your parking brake, let your car roll onto the brake then you should put your car into park. This will prevent damage to the parking pawl.
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