Main thing true for both squats is that you need to work on leading with your chest rather than your hips. Watch your transition out of the bottom position - your hips bounce up but your shoulders get left behind for a moment. When you transition out of the bottom, drive your head, shoulders and chest up and think about keeping your hips under the bar. You won't be perfectly vertical, but sometimes imagining that as your goal can help. As far as muscle weaknesses go, this is often just a posterior chain dominance over the quads, so get in a good volume of lighter squats in which you can stand back up w the proper posture and strengthen in. You may also have luck with pause squats.