S.H.
OK, so my family ran an early childhood education center in the 90s when time-outs were new. Here's the issue: time outs are not meant to be a punishment, but that's the way they are most commonly used. The technique of time out was introduced as a way to distract a child, get them to calm down and face the situation (or a new situation) with lower stress levels. If a child was doing something wrong, like jumping on the couch, the rule is say "get down" and the child is made to stop. MAKE the child stop. That means, pick her up and put her where you want her to be. Why should she get to decide if she obeys or not? I give my kids a chance, sometimes, but if it is clear that my words don't mean anything I use actions to show that they do. I am not talking about spanking (and I think hitting kids with a belt IS harmful. They know your hands control that belt, switch, spoon, whatever.) I am talking about lifting the child up over your head until they realize that the only person in control is you. And if they get upset, you sit them in time-out. You can sit next to them, you can hold their hand, you can help them calm down. That's what the purpose of time-outs was supposed to be.
Then once they are calm, a simple repititon of the rule is in order, or restitution for the offence, resolution of the fight, whatever. The idea is that after a time-out to cool down they will be able to face the issue calmly and perhaps rationally. In my experience, kids will do the right thing if they feel supported and calm.
Time out as a punishment is no more effective than the old putting your nose on the chalkboard or sitting in the corner. Not that it doesn't work for some kids and it is better than spanking if it does.
Try taking physical control of the situation when she does not stop. She'll know you mean business.
Good luck.