Dog won't poop, but will pee in box

We have a two-year old Lhasa Opso and a PetAPotty. We've had the potty for two months, and she hasn't pooed in it yet, but she pees in it several times a day. We have to take her outside for #2, or she'll do it on the carpet a few feet away from the potty.
There are times when we know she has to go BAD, and we set her on the potty, saying, "Go potty!" and she just sits there. We've sat there for 40 minutes once. Then, take her outside, and BAMM - she goes.
Anyone have any ideas on why she isn't going #2 on it?
(It used to have sod, now it has artificial grass, but she went on neither...)

Thanks!

i'd say your dog's pretty smart. one of my dogs is chained because he's too protective of me and bites. he will pee in one area and poo in another and refuses to step within that potty area unless he's pottying. so, your dog doesn't want to do 1 and 2 in the same spot. and he doesn't want to do it where he sleeps/is comfortable (your house.) so, he's conflicted. how young is he? if he's young or new to training, things may smooth out. if he has a kennel put him in it. lock him up after play time and in one or two hours let him outside. if he goes. PRAISE!

allison