Automatic Litterbox Central

I got a CatGenie for my two little Parson Russell Terrier mutts, so they won't have to hold their bladders and bowels until I can take them out (I imagine some people do this so they can neglect their dogs—I work at home and am happy to take them out whenever they need to go out, but I'd like them to be able to relieve themselves whenever they want without having to wait even a minute).

One dog (Poppy) has been using it successfully since day one ; the other dog (Posey) hasn't used it once since I installed it about 2 weeks ago .

Any suggestions?

Here's what I've tried:

-I tell them to "go potty" and reward them when they step into the CatGenie. Poppy sits there and then pees whenever she has to pee Posey just sits there, lies down, or hops out, and never pees

-I tried putting pee pads in the CatGenie, to get Posey to associate it with peeing, but no luck. I tried using a full pee pad, a partial pee pad, a soiled pee pad, a clean pee pad, a pee pad on a board on the CatGenie, and a pee pad on a towel on a board on the CatGenie. Nothing worked. She wouldn't even pee on any of these pee pads, even though she's uses pee pads just fine when they're elsewhere in the room.

-I tried spraying the litter with a spray that is supposed to make dogs want to pee where it's sprayed. No luck.

-I keep them in their crates inside of playpen whenever they're inside. I let them out of the crates every couple of hours. The only place to go is the CatGenie. Unfortunately, Posey has gotten so good at holding her bladder and her bowels, that she'll wait until walk time (I take them on a long walk, for about an hour) and manage to pee only during that time. Once or twice, she peed on the floor next to the CatGenie, or on her bed in the crate, but every other time, she's held it until we go on our daily walk.

I'm running out of ideas here.

Any suggestions, or at least experiences of fellow dog owners who've tried (either successfully or unsuccessfully) to get their dogs to use the CatGenie?

Thanks in advance for your help!