She's not steady to a thrown dummy because you are by your physical presence of standing in front of her or even with a light line physically preventing her from getting the dummy. I do it a different way and I can't claim to having the idea for it, it was a technique shown to me and it works. Dogs need to learn to be steady in their own head, not from outward forces....
I don't subscribe to the idea of tossing retrieves around and picking all bar one and letting the dog retrieve only that one or none either.... If the dog remains steady it gets every retrieve I throw otherwise you simply confuse the dog.....It has learnt that to get the dummy it has to remain steady and then you go tossing dummies about, it remains steady and doesn't get the reward for it.....What's that all about?

You end up with a dog that gets frustrated and probably breaks for it and then you probably do all that running it down stuff, scruffing it and taking it back to the place it broke the sit. If the dog reaches the dummy before you reach it then you are causing even more problems...
To do it as I was shown you need a dummy thrower....Sit the dog on a lead and have the dummy thrower toss a dummy close to the thrower. As the dummy hits the deck you give a light check on the lead and blow the stop quietly..If the dog remains sat, praise it. Then with the lead lose on the dogs neck drop the lead and put your foot on it...Have the thrower toss the dummy a short way and as it hits the deck blow the stop again if the dog remains sat praise it..If it doesn't it's not going to get the dummy so rinse and repeat...Most dogs remain sat....
Then go 'off lead' and have the dummy thrower toss the dummy a short distance from him/her again as it hits the deck blow the stop most keen retrievers will run in at this stage so the thrower should pick the dummy before the dog reaches it and tuck it away in a dummy bag well out of the dogs reach...You don't want the dog jumping at it at this stage either.....because to a high drive dog that is reward in itself...
Recall the dog rinse and repeat as soon as the dog remains steady send the dog on it's name....they pretty soon work out that to get that dummy it is conditional on their remaining seated.....If they run in they get nothing...Thereafter maintain it in different scenarios and if it breaks down go back a step you've moved too fast for the dog.....leading up to working two dogs...
Rascal