Housebreaking

I adopted a 2 year old female cocker, Marcie, from a rescue organization. In her previous life, she belonged to a breeder, but hadn't been bred yet. The breeder surrendered her because she decided not to breed anymore. She didn't have a lot of experience being around people, but once she warms up, she's as sweet and gentle as can be.

She wasn't housebroken when I got her. She was too big for my old cage, so I took her out every hour and watched her like a hawk. I have a doggy door that she uses. Now, she never goes in the house when I am not home. The problem is that we'll go outside and I'll tell her to go to the potty. She'll sniff around and/or lay in the garden. I'll keep telling her to go and she won't. WHen she has gone outside in front of me, I have praised her by telling that she's a good girl and giving her some scratches and pets. I tried rewarding her with treats, but she became so fixated on the treats that she wouldn't go. After 10-15 minutes, we'll go back inside and she'll go on the floor. She does the same thing at night because she sleeps with me and my other cocker in my bedroom. I'm now putting her in her a new, larger cage when she comes in if she doesn't relieve herself outside. I let her out every hour until she goes. She'll often wait 3-4 hours beforre going outside, whereas before she would pee within being back in the house for 10 minutes. Because she's housebroken when I'm not home and she won't go in front of me in the house, I'm assuming that there is some sort of underlying issue. Do any of you have any insight?