As everyone said go back to basics, she's only 9 1/2 months. It might be worth also only blowing the stop when you know you can stop her, ie put her in a sit, throw a ball out and stand between her and the ball. Ask her to fetch it and as she passes you try the stop whistle, that way the stop is close to you rather than some distance away and will allow you to correct it.
Sage is 15 months, her stops are pretty good now, but probably didn't start really kicking in until she was about 12 months.
Just take it one day at a time, the important thing is don't blow too many stops that she does wrong. If it's not working on a particular day, call it a day and don't give her the opportunity to get it wrong.
Look on the bright side though, at least she's coming back to you, which although it's not exactly what you want, it's still good