My trainer told me that when a dog truly knows something, such as leave it, it is a reaction no thinking involved on the dogs part. Do you clicker train? Instead of avoiding the ball situation what about training her to come to you when you see the start of the situation? To train this it will take some time, first you have to teach her to turn her head when you call her name, so if you clicker training you have a friend distracting your dog (when starting out the distraction is just the person, no treats, toys etc) the only 2 rules for the distraction person is don't use the dogs name and don't say come. You are very close maybe a foot or so away from the distraction person and the dog. YOu say "dogs name, come" as soon as the dog looks click, back away from the distraction person and give a treat. If the dog does not look given them a few seconds and have the distraction person stand up and ignore the dog, when the dog looks at you click, move back and treat. Do this a ton of times in 5 minutes then stop and train again later. When your dog gets really good at this distance up the space between you and the distraction. And do that for a while, then up the distraction to something wonderful. The point of this exercise is to teach your dog what do when you say leave it. If you say "leave it" " Fido come" she leaves the ball, other dog whatever knowing she is supposed to turn her head and come to you. This is a lot of information, let me know if you have any questions.
Start out easy so the dog will suceed. Still go to park, but maybe a different one with less dogs and balls? School yard when school is not in session? Good luck and keep us posted.