From an outsider perspective I'd say that she's not doing it because she doesn't really need to. Expectations of her are changing all the time, she's sometimes using a potty at nursery, and sometimes using a potty at home, sometimes in nappies, sometimes not- it's confusing!
As you've already sort of begun then I'd go for it full throttle, maybe over the weekend. The potty has been part of the confusion so if take it out of the equation and use the toilet.
Have a day when you can take her to choose and buy a toilet seat (or 2, depending on how many toilets she'll use at home) a step for each toilet and some brand new pants and stickers. Explain to her that the following day is toilet day, and that there will be no more nappies apart from at bedtimes.
Then, make sure you stick to it, don't ask her if she needs to go initially, take her every half hour ish. Run a tap while she sits to encourage weeing if you need to. If you have good success with few accidents then extend the time period to an hour etc (play this by ear). When there is a success make a big fuss, let her choose a relative to call (prep them in advance) and they can make a big fuss. Try and avoid sweets as rewards, they aren't necessary and there's many other things which can be rewards.
I think the key here is to re-establish the expectations and start again from fresh. Make it clear to her that all wees and poos need to go in the toilet now, and nothing in pants or on the floor. Get her excited about it, and keep it positive.
I had the same resistance with my dd and still do at age4.5.
We eventually put together a small bag of irresistible toys that she could only play with when sat on the potty. We called it Mr tumbles spotty potty bag and it worked a treat. Our had bubbles, a whistle, crayons and paper and other party bag type toys that she loved playing with. It was the novelty factor that helped because she knew what she had to do to get to play with that bag.
That's idea I mean we've got a seat my mum gave us, and have asked her a few times to use that but still no. I could try asking just about that instead. Obviously we don't punish her or make a big thing about not using it as that just makes the who situation worse, but it is a bit frustrating because i say "do you needs the potty, want to sit on it?" and she'll say no. Then 5 mind later, "look mummy I'm doing a wee on the floor" . Am always like never mind, you can try next time. But I'm quite close to thinking I'll stop altogether for a while. Didn't realise it would be this hard.....
My dd (three in three weeks) is in nursery for a few hours twice a week. Over the last month a few of them in nursery have started wearing pants and sitting on the potty at the same time with sticker charts etc. In preparation bought dd books ( pirate Peres potty etc), stickers for home chart, loads of pants etc and at first she was really excited and would sit on the potty at home every half hour or so, did a few wees and poos and we did the whole "wow" & stickers thing. But now she won't sit on it at home, she's still ok in nursery although has about 2 accidents in her session and won't ask to use it. But at home she always refuses to use the potty, we haven't pushed it, just left it there, kept asking and offered stickers, treats (& even sweets ) as a reward, none of which works. She hasn't done a wee or poo on the potty at home in over a week, just goes straight on the carpet everytime (which is smelling quite bad in this weather despite lots of cleaning). We've started putting nappies on again but still offering use of the potty regularly, always with the same answer "no!" I just don't know where to go from here really or what to try next!