No you are *not*a bad mum, its a frightening thing for a child to do poos on the loo or potty. DD1 refused for a long time, despite us trying every trick in the book. She just wasn't ready. Eventually we compromised, she was fine with weeing on the potty so i told her she could do all her wees on the potty and when it was time to do a poo, she could ask me for a nappy and do it there. She agreed and she then wore knickers all the time, and asked for a nappy when she needed to poo
We carried on like that for more than a year, she just had a fear and she would go rigid with resistance and fright if we tried to make her use the loo.
eventually her little sister started pooing on the potty, and dd1 saw that it wasn't such a big deal, that was just before christmas and we haven't looked back since.
All you can do is provide the opportunity, don't worry about what other kids are doing, just keep trying. I was always afraid to push too hard because I heard horror stories about kids holding it in, getting constipated and then being afraid of the pain as well. So we never pushed too hard, and we got there in the end.