3 1/2 yr old resistant to toilet training

[NZmum102]

My 3 1/2 year old daughter seems to be very resistant to toilet training. We have made some progress up to now, as she will now sit on the toilet, and will wee in the toilet, but just cannot seem to poo in it. We have just switched to knickers, so only use nappies at night now. But whenever she needs to poo, she either tells me and tries in the toilet with no success, or she won't tell me at all and a little bit will come out in her knickers. I'm worried that she is holding on and will start to get constipated. A similar thing happened about 4 months ago with the potty, and it became SUCH a big deal that it stressed us both out, and I just reverted back to nappies in desparation thinking I would try again once things had calmed down. But trying again just seems to have brought the same results... this time I am trying not to talk about it at all, and am letting her take responsibility for herself but I am desperate! All advice seems conflicting and I feel like we are swinging wildly from one 'fix' to the other, and nothing works. She is on pre-school holiday for the next two weeks, and I really just want to concentrate on getting this sorted so that she can go back to pre-school toilet trained! Someone, PLEASE help us....