I have a Pit/Mastiff mix and he took some time to learn as well. I know that he is food motivated so I gave him treats for going to the door, going potty outside, and coming when called. When giving the treats be excited which lets them know that you are happy and then she can associate you being happy with treats, which all dogs love.

Something that was previously stated is that they cannot hold there bladder that long. Even though my pup is now 8 months I still take him out every 30 min to and hour after eating and drinking which helped him to learn as well. Getting them on a schedule is very helpful. For example about 30 min after a meal, take her outside. When she goes, show her that you are excited and happy and reward her for good behavior. Every time you go out and she goes reward her.