Are you crate training? I have a 5month old and she has finally started to go to the back door when she needs to pee, which of course when she's in the house is every 20-30min!!! But for now, I will let her run out every 20-30min if that means she'll get the idea.
When I'm not home, she's in the crate. Sometimes when I'm home she's still in the crate, but that's usually after she's been running around awhile. She's just now started to have more "out of crate time". And when I am home she only has access to the rooms that I can see her from, everywhere else is baby-gated off. When I first brought her home she went to the bathroom in my guest room and then she would still try to get in there, so now it is TOTALLY off limits!! Keeping her within sight will also give you a better chance at catching her so she can be taken immediately outside. It is a bit annoying, as I'm sitting here I keep having to peek around the door frame to see what Issy's doing in the other room!! Some dogs, like kids, take a little longer. My first GSD never had an accident in the house, and I had him from 5weeks old, he was a dream!!! Last month, Issy would be running through the room playing keep away with Daphne and in mid stride would stop and pee!!! Good luck, she'll get it soon!