Oooh COD! (refering to post further down) I have a girl who was difficult in the poohing area... Wees were managed and mastered pretty quickly, but poohing - URGH! Took ages.
Frizbetheexpansionset - for what it's worth. It wasn't that she didn't like evacuating on the loo (although it was suggested that quite often small children didn't like the first feelings of what they saw as part of their body dropping away from them down the loo??!??!?! Possible perhaps, but not in our case). It was, we realised, because she had to do it somewhere private. She would always disappear behind the sofa, or another room.
Once we realised this, we explained (eventually) that we could leave her on her own on the toilet to pooh. Even now after a year, she will say (to grown ups and her sister, but not so much her friends ) "leave me alone and shut the door". Weeing is fine.
Other suggestions made to me at the time were to lay a nappy in the bottom of the potty or suspend one under the toilet seat (never managed that!)
And leave potties a plenty in those areas where she might be likely to go off and do one.
But I do remember her really clenching and it causing her considerable distress and pain whilst trying NOT to pooh in front of anyone. Poor wee thing.