Most kids have a problem with the pooping on the potty. Don't know why. With all three I've potty trained (the youngest at 19 months) I just had to watch and try to determine their bowel habits. When I suspected it was about that time I would make them sit on the potty and hopefully catch them at the right time. It is very time consuming but if you can get them to go once or twice they get over their fears. With my second child a little chocolate treat while sitting made all the difference. She couldn't wait to go poop on the potty to get a chocolate chip or two. But no snack for just sitting...there had to be some poop. The latest doesn't care about the treats. I just have to watch her very closely about the time she would usually go. I kept her confined in a small area of the house with the potty chair easily accessible. When I saw that familiar face I hurried and put her on the potty. (Oh and with all three we spent quite a bit of time running around with no pants on at all. I find that the sensation of pee or poop running down the legs is very disturbing to them and they will want to sit on the potty.)
She usually has two or three false alarms now and then manages to get on the potty for the poop. She's been doing really well for a month now but has had two poop accidents and pooped twice in her diaper in the middle of the night. I just keep reminding her that poop goes in the potty about that time in the afternoon (like every ten minutes) and that we don't go poopy in our panties. She's pretty young so the reminders are necessary, but the other two were about 2 1/2 and they didn't need it that often.
Don't give in though. You let them talk you into pull-ups to poop and they will be forever be doing that. You just have to insist that they wear their big boy pants or big girl panties...and if they have an accident just gently scold them and tell them you're disappointed. Take them to the potty and show them the poop goes in the potty by emptying out the pants in the potty and flushing. Clean them up and keep gently telling them "you don't go poopy in your pants" and ask them over and over "where does the poopy go?" They will get it. It takes longer than the whole pee pee deal...not sure why. You would think that it would be easier to determine you have to go poop than it would to know you have to pee.
Again...no compromise. If you compromise with the poop, they will be forever wanting to use a pull-up or diaper to poop.
And from my experience and those of a few friends...pull-ups are a waste of time when training. Those are just pull on diapers. My girls could care less. At home we wore the teri cloth panties and I kept a couple cans of carpet cleaner handy (after a week of running around naked). I would even have them help me clean up the mess. When we went out I might put a diaper on them at first but I would keep stressing that though they had a diaper they still must tell me they had to go potty. I never wasted my money on pull-ups after the first week of trying with my oldest.
I work with three year olds and I've seen little ones wearing pull-ups for a whole year and still not have a clue what it means to go on the potty. Honestly, I think if they don't start to get the hang of it after a week, they aren't ready....(now that is if you are home with them and it is an all day affair trying to learn...if they spend most of their time in daycare...then it will take considerably longer to learn)
Hope you can get an idea or two from my posting and good luck!!! All mommy's that have potty trained know what you are talking about.