The answer to your question is contained in this statement. You don't need to add bunch of pressure to get this corrected. Work on leash with a retrieve as a reward. Do it up close at first then move her away from you. When she sits right, she gets the bumper, otherwise no reward. This doesn't have to be in a pile work setting. You can fake a throw and get her to move away from you then toot the whistle and either reward or not based on the response. Also look CLOSELY at the rest of this dog's life and I'll be you are tolerating a crooked sit in other places, when it's not critical. So.. what does sit mean? sit means feet square under you, mouth closed (no noise), eyes on me! That's the only thing that get rewarded. Helps if you started this from the beginning but since you didn't, you can correct it now. It'll take time but it will work if you clean up your training and be consistent. More pressure may cause all kinds of other issues from lack of eye contact (avoidance) to laying down to who knows what. Do some close distance positive work on it and see if you can't change her overall habit from there.
Once she's doing it right and I mean REALLY, consistently right, give a meaningful correction if she gets lazy. Don't nag nag nag with come in whistles and nicks.
Just my take, for what it's worth (which is exactly what you paid).