4 1/2 month old Cairn Female won't potty outside

[Tina2020]

Hello, 

our 4 1/2 month old female Cairn puppy has a problem with peeing/pooping outside. We take her out very often, every couple of hours, after naps, after she eats, plays. We can walk her or stand in one place to let her sniff the area (spot where she usually pees in the morning) for a long time and she still won't do it. We come home and she does it on a wee wee pad. There are days where she takes care of her business outside but we can count those on one hand. She usually pees outside in the morning. We live in a city, don't have an access to a garden. She does use wee wee pads. We take her to the park to run around, play with other dogs and she still comes home and does it on the wee wee pad. We had an Australian terrier (male) before and we had no issues whatsoever, he knew really quickly that he has to take care of his business outside. We praise her when she pees outside, we give her her favorite treats. Do you fellow Cairn parents have any advice? Please help! Thank you!!