M.G.
Bribery!! A long time ago a friend of mine was have a terrible time with her little girl. She'd been pee trained for a while, but just refused to poop on the potty. She got to the point where she was just holding her poop and causing problems. I bought a little present for Julia and wrapped it in rather generic party paper. I dropped it off at her house and told her mama to use it as bribery. It work. She knew that if she pooped on the potty she'd get that present and it took about a day or so, but she did it!! It was a very small present. I also use little treats to potty train my daycare kiddos: M & Ms, Smarties, etc. They know that if they do what is expected of them they'll get a treat.
Good Luck.