Selective poo withholding

[eleanormul]

My three year old daughter has been potty trained for a couple of months now. She's absolutely fine with wees but poos are another matter. At her nursery, etc. She's fine and also when at her grandparents but at home we regularly get into a pattern when she tells me she needs to poo, we go to the toilet and nothing happens. This happens over and over and over until she goes in her pants. She talks about it being sore but the poos that come out don't seem overly hard or large. It's not a constant thing. But it is regular. She's pretty strong willed and we've had regular battles over other things too like toothbrushing. I find the whole situation incredibly frustrating and though I try not to show this I regularly fail. Any advice?