I can't believe how timely your question is for me. I have been potty training my 32-month old for just over two weeks now. He mostly has the peeing down, and has very few accidents anymore, but he has not pooped in the potty once. Well, that's not exactly true. A few days ago I realized he needed more incentive and bought him a couple of Thomas trains and some matchbox cars for him to earn by pooping in the potty. He spent the whole morning wanting to try but then giving up when he sat on the potty because it was too hard (literally...since he hadn't pooped for 3 days...) Finally he did it, I gave him the first train, and then he did some more in his undies. So I took the train away. Bad parenting move? I didn't know what else to do, since I needed him to understand that when you poop, you need to make sure you get it all out. And the whole point here is that we don't go in our underwear!
Anyway, so yeah. He is holding it in until it hurts to go, and then doesn't want to go at all. It's been a few days now since I bought the trains, and he still hasn't earned a single one, even though he talks every day about earning his trains, so I know he gets it. I have let him watch me poop. And I do try to catch him just before he goes, but man, he is quick about it. As soon as I see him crouch down, the poop is out. And it doesn't matter if I asked him 2 minutes earlier if he needed to go, or even if I sat him on the potty.
Don't know if this is relevant, but I took the little potty away before we started potty training, because if he could use that, he would never go in the big potty, and I needed him to be able to use the toilets in public places and other people's houses. I'm wondering if I get the little potty back out, maybe he will poop in there? Because he had done that a few times before we started training. And as much as I hate cleaning that out, it would be better than his underwear. :)
Sorry for not helping, I am just as mystified as you over what to do next. I guess I'll keep checking back for advice myself. :)