Poo training children...???

Has anyone had problems with number 2 training their child?

My DD is 3 and has been wee trained since 2 1/2 and is quite happy to take herself off to the toilet for a wee but we have only had a few weeks of number 2 success's?
She holds on to poo's and used to do this until night time when she would have a night nappy on. Now she continuously squeezes her bottom cheeks but will have lots of accidents in her knickers (can be up to 7 pairs of knickers a day). I dont get cross with her and just say nothing and clean and change her. We have tried sticker charts and rewards of sweets etc once there is a poo on the potty. I try to catch her on her strain of holding it in and put her straight on the potty and very occasionally she will do a decent sized poo then we go mad on the praise etc. She is not happy about sitting on the toliet but will sit for a decent period of time on a potty. I try to give her plenty of time without any knickers/trousers etc on to make it easier for her. I feel a bit of a failure on this as she picks up everything else we teach her so quickly. I ask her regulary if she need to use the potty but her response to us is that "Its OK mummy I do it in my knickers!"

Where am I going wrong?
Will I ever get her poo trained?

I really would appreciate any suggestions as I really dont know what to do now?