My daughter is 3.5 & knows how to use potty but refuses. What should I do and what age should I worry? We try not to shame or push her.

Rewards. Always try to be happy when she goes in toilet.You should know about when she needs to poop and taking her to the toilet then may help her get the picture.You are right not to push or shame.Are the stools hard or very large?This may point to a problem and best discussed with your dr.
Potty train Shame. First it's great you say that you don't shame ; never shame or humiliate a child into doing anything. Simply takes time. Here are what seem to be some good tips: http://www.Babycenter.Com/0_successful-potty-training-for-girls_11653.Bc luck.