I have two adult cats, a male and a female. We live in an apartment and I like to let them outside when I sit in our little backyard. The male comes back on his and asks to be let back inside, but the female explores off into the treeline. I whistle to call them if I lose track of them, and the male comes to me immediately, but the female only comes sometimes when she knows she's gone too far out, but always runs past me to the other side and wanders off again. I give them a treat after to let them know it's the behavior I want. The male has taken to it very well, but the female is stubborn and I often have to go get her and carry her back inside. What can I do to get the female on board with this?